What is thyroid hormone therapy?

On this page

• Overview
• What is the difference between levothyroxine and liothyronine?
• What do guidelines and trials show about thyroid hormone therapy?
• What does research show about Armour Thyroid compared with levothyroxine?
• What does research show about thyroid hormone for aging and energy?
• Is thyroid medicine used for weight loss?
• Potential benefits
• Limitations
• Risks and side effects
• What changes with thyroid medicine during pregnancy?
• Who should not take thyroid hormone?
• How do you take levothyroxine?
• What happens after you start levothyroxine?
• How long until results?
• What has FDA said about levothyroxine and liothyronine?
• What has FDA said about Armour Thyroid and other desiccated thyroid products?
• Are compounded thyroid medicines FDA-approved?
• Do you need a prescription for thyroid medicine?
• How much does thyroid hormone therapy cost?
• Does insurance cover levothyroxine?
• What should I ask my doctor before starting thyroid hormone therapy?
• Frequently asked questions
• References
• Page information

Overview

The American Thyroid Association's 2014 guideline recommends levothyroxine as the preparation of choice for hypothyroidism, citing its effect on symptoms, long experience of its benefits, a favorable side effect profile, ease of use, good absorption, a long half-life in the blood and low cost.[2]

We did not review the trials behind the guideline statement that levothyroxine resolves the symptoms of hypothyroidism, so that statement is reported on this page as guideline content.

Primary hypothyroidism means the thyroid gland itself is underactive; in secondary and tertiary hypothyroidism the cause lies in the pituitary gland or the hypothalamus.

What is the difference between levothyroxine and liothyronine?

Levothyroxine is a synthetic form of thyroxine (T4), and liothyronine is a synthetic form of triiodothyronine (T3).[1][2]

According to the Synthroid prescribing information, the effects of thyroid hormones come mainly from T3, and about 80% of the body's T3 is made from T4 in tissues outside the thyroid gland.[1]

Sources: Synthroid tablets prescribing information, Revised 2/2024.

What do guidelines and trials show about thyroid hormone therapy?

Guidelines for hypothyroidism

Levothyroxine doses in primary hypothyroidism are adjusted to bring thyroid-stimulating hormone (TSH) into the normal reference range, according to the 2025 European Thyroid Association guideline.[4][1]

For people already taking levothyroxine for primary hypothyroidism, the 2025 European Thyroid Association guideline says that once the target TSH is reached, complicated regimens and very small dose changes made to improve quality of life or to change body weight are not useful and not advised.[4]

The 2025 European Thyroid Association guideline recommends a more relaxed TSH reference range for people over age 70 who take levothyroxine for hypothyroidism, and rates that recommendation as weak.[4]

The American Thyroid Association's 2014 guideline lists avoiding overtreatment, especially in older people, among the goals of levothyroxine treatment.[2]

Subclinical hypothyroidism in older adults

Subclinical hypothyroidism means a raised TSH level with a free thyroxine (free T4) level that is still in the normal range.[5]

In the TRUST trial, 737 adults aged 65 and older with subclinical hypothyroidism (TSH between 4.60 and 19.99 mIU/L) took levothyroxine or a placebo, and after one year levothyroxine had lowered TSH but had not improved hypothyroid symptoms or tiredness compared with placebo.[6]

On two 100-point scales, where the TRUST trial counted 9 points as the smallest difference that matters to patients, the levothyroxine and placebo groups differed by 0.0 points for hypothyroid symptoms and 0.4 points for tiredness after one year.[6]

The TRUST authors concluded that "Levothyroxine provided no apparent benefits in older persons with subclinical hypothyroidism."[6]

The TRUST trial reported no significant excess, in the levothyroxine group, of the serious adverse events it had prespecified as being of special interest.[6]

TRUST lasted one year, the average starting TSH was 6.40 mIU/L, and the trial was funded by the European Union's FP7 research program and others.[6]

A 2018 systematic review in JAMA of 21 randomized trials, with 2,192 adults who had subclinical hypothyroidism and were not pregnant, found that thyroid hormone therapy lowered TSH into the normal range but was not associated with benefit for general quality of life (measured in 796 people) or for thyroid-related symptoms (measured in 858 people).[7]

The authors of the 2018 JAMA review judged the overall quality of the evidence to be moderate to high and wrote that their findings "do not support the routine use of thyroid hormone therapy in adults with subclinical hypothyroidism."[7]

The authors of the 2018 JAMA review cautioned that their results may not apply to people older than 80.[7]

A 2019 analysis in JAMA combined 251 adults aged 80 and older with subclinical hypothyroidism from two placebo-controlled trials, 112 given levothyroxine and 139 given placebo, and found no significant difference in hypothyroid symptoms or tiredness after one year.[8]

Authors of the analysis in adults aged 80 and older reported grants from research funders including ZonMw, the European Union's FP7 program and the Swiss National Science Foundation, and some authors reported nonfinancial support from the drug company Merck KGaA.[8]

Guidelines on subclinical hypothyroidism were written in different years and use different TSH thresholds.

A 2019 BMJ guideline panel made a strong recommendation against thyroid hormones for adults with subclinical hypothyroidism, and said the recommendation does not apply to women who are trying to become pregnant or to people with a TSH above 20 mIU/L, and may not apply to people with severe symptoms or to young adults such as those aged 30 or younger.[5]

The European Thyroid Association's 2013 guideline, written before the TRUST trial, recommends levothyroxine for people younger than 65 whose TSH is above 10 mU/L even if they have no symptoms, and a wait-and-see approach that generally avoids hormone treatment for people older than 80 to 85 whose TSH is 10 mU/L or lower.[9]

For people younger than 65 who have symptoms suggestive of hypothyroidism and a TSH below 10 mU/L, the European Thyroid Association's 2013 guideline says a trial of levothyroxine should be considered.[9]

The European Thyroid Association's 2025 guideline says that starting levothyroxine in older patients with subclinical hypothyroidism and a TSH between 7 and 10 mU/L should be tailored to the patient and must take heart and bone conditions into account.[4]

Do not change or stop a prescribed medicine without talking with the clinician who prescribed it.

Adding T3 to levothyroxine

A 2021 systematic review that included 18 trials with 1,563 adults with hypothyroidism found no difference between levothyroxine plus liothyronine and levothyroxine alone in clinical status, quality of life, psychological distress, depressive symptoms or fatigue, and rated the certainty of that evidence as low to moderate.[10]

A 2021 consensus document from the American, British and European thyroid associations said that most of the 14 clinical trials it counted did not support combination therapy over levothyroxine alone, and that a few trials showed overall benefits for mood, quality of life and neurocognitive function.[11]

The 2021 systematic review of 18 trials also pooled patient preference: 43% of participants chose combination therapy, 23% chose levothyroxine alone and 30% had no preference. Preference is a different outcome from symptoms or quality of life, where the review found no difference.[10]

In a 2021 crossover trial of 75 people with hypothyroidism, an exploratory analysis of the third who had the most symptoms on levothyroxine found a strong preference for treatments containing T3 and better scores on symptom, quality of life, depression and visual memory measures; for the 75 people as a whole the trial found no differences in its main outcomes.[12]

The European Thyroid Association's 2012 guideline says there is insufficient evidence that levothyroxine plus liothyronine serves people with hypothyroidism better than levothyroxine alone, and recommends that levothyroxine alone remain the standard treatment.[3]

The European Thyroid Association's 2012 guideline says combination therapy might be considered as an experimental approach for people who take levothyroxine as directed and still have persistent complaints despite a TSH within the reference range, and suggests stopping it if there is no improvement after 3 months.[3]

The American Thyroid Association's 2014 guideline recommends against the routine use of levothyroxine plus liothyronine for primary hypothyroidism, citing conflicting results from randomized trials and too little long-term outcome data.[2]

Wilson's syndrome

The American Thyroid Association's 2014 guideline describes Wilson's syndrome, also called Wilson's temperature syndrome, as having the hallmarks of fatigue, anxiety, depression, headaches, insomnia and muscle aches, with a low body temperature as the feature that sets it apart from chronic fatigue syndrome.[2]

The American Thyroid Association says it "has found no scientific evidence supporting the existence of" Wilson's syndrome, in a statement dated 2005 and updated in 2016.[13]

The American Thyroid Association's 2014 guideline says there are no credible scientific data to support the existence of Wilson's syndrome and recommends against T3 escalation therapy for it, citing a lack of proven benefit and the risk of thyroid hormone excess.[2]

In searches of PubMed, Europe PMC and ClinicalTrials.gov on October 1, 2026, we found no controlled trial of T3 for Wilson's syndrome among the records we screened.

Among the reports of treated people that those searches returned, one concerns T3 treatment connected to Wilson's temperature syndrome: a 2006 report of 11 people treated for chronic fatigue syndrome with a sustained-release T3 protocol.[14] Its abstract describes no comparison group and says all 11 improved on the five symptoms measured.[14] A report with no comparison group cannot show that the treatment caused the improvement, and we read only the abstract.[14] The searches also returned a 1999 clinical guide whose title concerns T3 therapy for Wilson's syndrome; it has no abstract and we did not read it.

What does research show about Armour Thyroid compared with levothyroxine?

Desiccated thyroid, also called thyroid extract, is made from dried pig thyroid glands and contains both T4 and T3.[2]

Synthroid (levothyroxine) and Cytomel (liothyronine) tablets are FDA-approved prescription drugs, taken by mouth, for hypothyroidism and certain other labeled uses; FDA says animal-derived thyroid medications, such as Armour Thyroid, are not FDA approved (checked October 14, 2026).

Two small randomized crossover trials that we read, one with 70 participants and one with 75, compared desiccated thyroid extract with levothyroxine in adults already taking levothyroxine for hypothyroidism.[15][12] The first found no differences in symptoms or neurocognitive measurements, and the second found no differences in its primary or secondary outcomes apart from a minor increase in heart rate on desiccated thyroid.[15][12]

The 70-person trial gave each treatment for 16 weeks, and the two trials shared authors.[15][12]

In the 70-person trial, 34 participants (48.6%) preferred desiccated thyroid, 13 (18.6%) preferred levothyroxine and 23 (32.9%) had no preference, and participants weighed about 3 pounds less at the end of the desiccated thyroid period than at the end of the levothyroxine period.[15]

The 75-person trial found no difference in treatment preference, and the 3-pound difference comes from people with hypothyroidism over 16 weeks, so it says nothing about weight loss in people with normal thyroid function.[12][15]

The Synthroid and Cytomel labels carry a boxed warning that thyroid hormones should not be used to treat obesity or for weight loss; neither is FDA-approved for weight loss (checked October 14, 2026).

In a randomized, double-blind trial sponsored by Allergan, the maker of Armour Thyroid, 284 adults whose hypothyroidism was controlled on levothyroxine were assigned to Armour Thyroid (141 people) or to levothyroxine (143 people); 60.76% of the Armour Thyroid group and 75.92% of the levothyroxine group had a TSH in the normal range of 0.45 to 4.12 mIU/L at the end of both the dose-adjustment period and the stabilization period.[16]

The difference between the Armour Thyroid and levothyroxine groups was 15.16 percentage points in favor of levothyroxine, with a 95% confidence interval of 4.23 to 26.10 percentage points.[16]

The Armour Thyroid trial results come from the trial's ClinicalTrials.gov record, which does not state the non-inferiority margin, and TSH is a blood test result, not a measure of how people felt.[16]

The American Thyroid Association's 2014 guideline recommends levothyroxine in preference to thyroid extracts and says high-quality, controlled, long-term outcome data showing that thyroid extracts are superior to levothyroxine are lacking.[2]

The American Thyroid Association's 2014 guideline puts the ratio of T4 to T3 in desiccated thyroid preparations at 4.2 to 1, compared with 14 to 1 in what the human thyroid gland secretes.[2]

The European Thyroid Association's 2012 guideline says combination tablets, including those containing animal thyroid extract, are potentially harmful because their relatively high T3 content carries a risk of inducing symptoms of thyroid hormone excess.[3]

In a statement dated September 18, 2025, the American Thyroid Association said synthetic levothyroxine is the standard therapy for hypothyroidism, and that desiccated thyroid extract is not recommended as first-line therapy although the association recognizes that it is the treatment of choice for some patients.[17]

What does research show about thyroid hormone for aging and energy?

Symptoms with normal thyroid tests

In searches of PubMed, Europe PMC and ClinicalTrials.gov on October 1, 2026, among the records we screened, a placebo-controlled trial we found in people who had symptoms of hypothyroidism but thyroid tests in the normal range was a 2001 crossover trial in 25 people (22 completed it); it found thyroxine no more effective than placebo for cognitive function or psychological wellbeing after 12 weeks of each.[18]

In the 2001 crossover trial, 19 healthy volunteers scored significantly lower for vitality while taking thyroxine than while taking placebo, an average of 60 compared with 73.[18]

In searches of PubMed, Europe PMC and ClinicalTrials.gov on October 1, 2026, we found no other randomized trial of thyroid hormone in people without a thyroid disease diagnosis that measured how tired or energetic they felt, among the records we screened.

We read only the abstracts, not the full texts, of three trials in Antarctic residents (Palinkas 2007, Palinkas 2010 and Reed 2001) that report mood or cognitive measures; Reed 2001's abstract does not say the trial was randomized.[19][20][21]

Records of trials and reviews that added thyroid hormone to treatment for depression or bipolar disorder were screened by title only, registry records were read by title, condition and intervention only, and the broad unfiltered searches were screened only in part.

The 2001 trial's lead author held a scholarship from the Association of Clinical Biochemists, and the authors declared no competing interests.[18]

The American Thyroid Association's 2014 guideline strongly recommends against levothyroxine for people who have nonspecific symptoms and normal thyroid blood tests.[2]

Aging and longer life

In searches of PubMed, Europe PMC and ClinicalTrials.gov on October 1, 2026, we found no trial that gave thyroid hormone to healthy adults to test whether it slows aging, lengthens life or lowers a measure of biological age, among the records we screened.

In the Leiden 85-Plus Study, an observational study that followed 599 people in the Netherlands from age 85 through age 89, higher TSH levels were associated with a lower death rate.[22]

The Leiden study shows an association in people aged 85 and older; it does not show that taking or not taking thyroid hormone changes how long people live, and its authors wrote that evidence for not treating older people "can only come from a well-designed, randomized placebo-controlled clinical trial."[22]

The thyroid hormone products whose labels we reviewed, Synthroid (levothyroxine) and Cytomel (liothyronine) tablets, are FDA-approved for hypothyroidism and, with surgery and radioiodine therapy, to suppress TSH in certain thyroid cancers (Cytomel also as a diagnostic suppression test), not for anti-aging, longevity or energy. Prescribing these products for anti-aging, longevity or energy is off-label. FDA has not reviewed whether it is safe and effective for that purpose.

Is thyroid medicine used for weight loss?

A 2009 systematic review of thyroid hormone for obesity called the available data "inconclusive" on whether the treatment helps, and said the data support that such treatment induces subclinical hyperthyroidism, a mild excess of thyroid hormone seen on blood tests; the review's authors noted that the usable studies were few and small.[23]

The American Thyroid Association's 2014 guideline recommends against using liothyronine to treat obesity in people with normal thyroid function, citing a lack of controlled data showing that it works.[2]

The thyroid hormone products whose labels we reviewed, Synthroid (levothyroxine) and Cytomel (liothyronine) tablets, are FDA-approved for hypothyroidism and, with surgery and radioiodine therapy, to suppress TSH in certain thyroid cancers (Cytomel also as a diagnostic suppression test), not for weight loss. The Synthroid label's boxed warning, for example, says: 'Thyroid hormones, including SYNTHROID, either alone or with other therapeutic agents, should not be used for the treatment of obesity or for weight loss.' Prescribing these products for weight loss is off-label.

The Synthroid prescribing information carries a boxed warning, 'WARNING: NOT FOR TREATMENT OF OBESITY OR FOR WEIGHT LOSS'. It states: 'Thyroid hormones, including SYNTHROID, either alone or with other therapeutic agents, should not be used for the treatment of obesity or for weight loss. In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction. Larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given in association with sympathomimetic amines such as those used for their anorectic effects'.

Sources: Synthroid tablets prescribing information, Revised 2/2024.

Potential benefits

Evidence levels in this table follow our grading key: Established means consistent results from randomized trials or a systematic review of them, and 'as guideline content' means only that a guideline says so; Emerging means some controlled human evidence that falls short of that, such as small or short trials, mixed results or trials funded by a maker; Experimental means we found no controlled human study, only reports without a comparison group or laboratory or animal work; Evidence against means controlled trials tested the claim on its main result and did not find it; and 'no support found' means our search found none. In this table, Not yet checked by us means sources exist that we have not reviewed. An evidence level describes how much research exists and what it found; a level in this table is not a recommendation to use thyroid hormone for that purpose.

The Synthroid and Cytomel labels carry a boxed warning that thyroid hormones should not be used to treat obesity or for weight loss; neither is FDA-approved for weight loss (checked October 14, 2026).

Neither the Synthroid nor the Cytomel label lists anti-aging, longevity or energy as an approved use; both are approved for hypothyroidism and certain other labeled uses (checked October 14, 2026).

Limitations

Several studies cited on this page were read as published abstracts, so details that appear only in the full papers are not reported here.

We did not search for trials that treated people with normal thyroid tests to a narrower TSH target, so this page reports no evidence for or against that approach.

Risks and side effects

The Synthroid prescribing information says the side effects of levothyroxine are mainly those of an overactive thyroid caused by too high a dose.[1]

The side effects listed in the Synthroid prescribing information include palpitations, a fast heart rate, irregular heart rhythms, tremors, nervousness, insomnia, heat intolerance, excessive sweating, weight loss, diarrhea, hair loss, menstrual irregularities and decreased bone mineral density.[1] The Synthroid prescribing information also lists angina, heart failure, heart attack and cardiac arrest among the reactions that come with too high a dose.[1]

The Synthroid prescribing information describes levothyroxine as having a narrow therapeutic index and says that overtreatment or undertreatment may have negative effects on the heart and circulation, bone, reproductive function, thinking, digestion, and sugar and fat metabolism.[1]

The Synthroid prescribing information says atrial fibrillation is the most common irregular heart rhythm seen with levothyroxine overtreatment in older people.[1]

The Synthroid prescribing information says overtreatment may bring on angina or irregular heart rhythms, particularly in older people and people with cardiovascular disease, and calls for lower starting doses in those groups.[1]

The Synthroid prescribing information says levothyroxine over-replacement may increase bone resorption and lower bone mineral density, particularly in post-menopausal women, and advises the minimum dose that achieves the desired response.[1]

A 2026 systematic review of 12 studies with 1,817 participants compared levothyroxine with placebo, no treatment or other thyroid hormone regimens. It reported that most comparisons of side effects were not statistically significant, including heart effects such as atrial fibrillation, effects on the nervous system and mood, and muscle pain, and that side effects occurred mainly when TSH was suppressed. The review recorded TSH values to describe each group's thyroid status as euthyroid (normal thyroid tests), TSH suppressed or hypothyroid. By its methods it included people with hypothyroidism, myxedema, thyroid cancer or Hashimoto's thyroiditis who were treated with levothyroxine. Its study table also lists people with subclinical hypothyroidism and people with thyroid nodules, and a group of women with multinodular thyroid disease and normal thyroid function whose doses suppressed TSH. The review does not show how levothyroxine affects people with normal thyroid tests who have no thyroid disease.[24]

Studies of TSH levels, doses and later health problems

In a cohort study of 17,684 people taking thyroxine in Tayside, a fully suppressed TSH (0.03 mU/L or lower) was associated with more cardiovascular disease (hazard ratio 1.37), more heart rhythm problems (1.6) and more fractures (2.02) than a TSH in the normal range, while a TSH that was low but not suppressed was not; a high TSH was also associated with all three.[25]

In a study of 213,511 levothyroxine users aged 70 and older in Ontario, in which 22,236 (10.4%) had a fracture over an average of 3.8 years, current levothyroxine use was associated with a higher fracture risk than use in the distant past (adjusted odds ratio 1.88), and among current users high and medium doses were associated with a higher risk than low doses (3.45 and 2.62).[26]

In the Ontario study, 88% of the people with fractures were women, and a higher levothyroxine dose may mark a different underlying condition, so the study cannot show that the dose caused the fractures.[26]

In a study of 705,307 US veterans taking thyroid hormone (88.7% men, median age 67), a TSH below 0.1 mIU/L and a TSH above 20 mIU/L were each associated with a higher risk of cardiovascular death than thyroid tests in the normal range, with adjusted hazard ratios of 1.39 and 2.67.[27]

In a retrospective cohort study of 21,035 adults taking levothyroxine, published in the journal Military Medicine in 2025, 1,091 (5.2%) developed atrial fibrillation; the starting TSH level was not significantly associated with atrial fibrillation, while people in the highest quarter of free T4 levels had a higher rate than those in the lowest quarter (hazard ratio 1.22, adjusted for age and sex).[28]

Observational studies of people taking thyroid hormone show associations between TSH levels or doses and later health problems; they cannot prove that the dose or the TSH level caused those problems.

How often are thyroid levels checked on levothyroxine?

The Synthroid prescribing information says to check TSH 6 to 8 weeks after any change in dose in adults with primary hypothyroidism.[1]

For adults on a stable dose, the Synthroid prescribing information says to review symptoms and blood tests every 6 to 12 months and whenever the person's clinical status changes.[1]

The Synthroid prescribing information says biotin supplements may interfere with thyroid blood tests and advises stopping biotin at least 2 days before TSH or T4 is measured.[1]

Questions about dose, side effects or other medicines are for the licensed clinician who manages the thyroid hormone treatment.

The Synthroid and Cytomel labels carry a boxed warning that thyroid hormones should not be used to treat obesity or for weight loss; neither is FDA-approved for weight loss (checked October 14, 2026).

Sources: Synthroid tablets prescribing information, Revised 2/2024.

If you have severe symptoms, call 911 or seek emergency care. Report side effects to FDA MedWatch.

What changes with thyroid medicine during pregnancy?

The Synthroid prescribing information says pregnancy may increase the levothyroxine dose a person needs.[1]

For pregnant patients who already have hypothyroidism, the Synthroid prescribing information says to measure TSH and free T4 as soon as pregnancy is confirmed and at least once during each trimester.[1]

The Synthroid prescribing information says the levothyroxine dose should return to the pre-pregnancy dose immediately after delivery.[1]

The Synthroid prescribing information says levothyroxine should not be stopped during pregnancy, that hypothyroidism diagnosed during pregnancy should be treated promptly, and that untreated hypothyroidism in the mother may have an adverse effect on the baby's neurocognitive development.[1]

Sources: Synthroid tablets prescribing information, Revised 2/2024.

The 2025 European Thyroid Association guideline recommends checking TSH, free T4 and total T4 at least once in each trimester in pregnant women who take levothyroxine for hypothyroidism.[4]

We did not read the recommendations of the American Thyroid Association's 2026 guideline on thyroid disease in pregnancy, so this page does not report them.

Who should not take thyroid hormone?

The Synthroid prescribing information lists uncorrected adrenal insufficiency as a contraindication.[1]

The Synthroid prescribing information says that starting thyroid hormone before glucocorticoid treatment in a person with adrenal insufficiency may bring on an acute adrenal crisis, and calls for glucocorticoid replacement first.[1]

The Synthroid prescribing information says that adding levothyroxine in people with diabetes may worsen blood sugar control and raise the amount of diabetes medicine or insulin needed.[1]

The Synthroid prescribing information says levothyroxine increases the response to oral anticoagulants (blood thinners), so a lower anticoagulant dose may be warranted and clotting tests are monitored closely.[1]

The Synthroid prescribing information says thyroid hormone taken by mouth is not recommended for myxedema coma, a life-threatening emergency that is treated with intravenous thyroid hormone.[1]

Sources: Synthroid tablets prescribing information, Revised 2/2024.

Whether thyroid hormone is appropriate for a particular person is a decision for a licensed clinician who knows that person's test results, other conditions and medicines.

How do you take levothyroxine?

The Synthroid prescribing information says to take the tablet once a day on an empty stomach, one-half to one hour before breakfast.[1]

The Synthroid prescribing information says to take the tablet at least 4 hours before or after medicines known to interfere with its absorption, and names iron supplements, calcium supplements and antacids among them.[1]

The Synthroid prescribing information says soybean flour, cottonseed meal, walnuts and dietary fiber may decrease the absorption of levothyroxine, and that grapefruit juice may delay its absorption and reduce how much is absorbed.[1]

The Synthroid prescribing information says dosing must be individualized, and lists age, body weight, cardiovascular status, other medical conditions including pregnancy, other medicines and food among the factors.[1]

Sources: Synthroid tablets prescribing information, Revised 2/2024.

For adults with hypothyroidism, the Synthroid prescribing information gives a full replacement dose of 1.6 mcg per kilogram of body weight per day and notes that some patients require a lower starting dose.[1]

In the Synthroid prescribing information, the dose for adults with hypothyroidism is adjusted in steps of 12.5 to 25 mcg every 4 to 6 weeks as needed, and for older adults and adults with heart disease or at risk of atrial fibrillation it is adjusted every 6 to 8 weeks as needed.[1]

Dose figures on this page summarize the Synthroid prescribing information and are not instructions to follow.

A prescriber sets and adjusts the dose.

Sources: Section 2 (Dosage and Administration) of Synthroid tablets prescribing information, Revised 2/2024.

The instructions on this page come from the prescribing information for Synthroid tablets; other levothyroxine products, liothyronine and desiccated thyroid have their own labeling, which may differ.

What happens after you start levothyroxine?

The Synthroid prescribing information says the dose for adults with primary hypothyroidism is adjusted until the person has normal thyroid function on clinical assessment and the TSH level returns to normal.[1]

The Synthroid prescribing information advises a lower starting dose for older adults, for adults with underlying heart disease and for adults at risk of atrial fibrillation.[1]

Sources: Synthroid tablets prescribing information, Revised 2/2024.

How long until results?

The Synthroid prescribing information says the peak effect of a given levothyroxine dose may not be reached for 4 to 6 weeks.[1]

Sources: Synthroid tablets prescribing information, Revised 2/2024.

We did not review trials that measured how quickly hypothyroid symptoms improve after starting levothyroxine, so this page gives no figure for that.

What has FDA said about levothyroxine and liothyronine?

Synthroid (levothyroxine) and Cytomel (liothyronine) tablets are FDA-approved prescription drugs, taken by mouth, for hypothyroidism and certain other labeled uses; FDA says animal-derived thyroid medications, such as Armour Thyroid, are not FDA approved (checked October 14, 2026).

The FDA status table lists the thyroid hormone products whose labels we reviewed, with how each is taken and the uses in its prescribing information.

It does not cover every levothyroxine or liothyronine product.

The Synthroid prescribing information carries a boxed warning, 'WARNING: NOT FOR TREATMENT OF OBESITY OR FOR WEIGHT LOSS'. It states: 'Thyroid hormones, including SYNTHROID, either alone or with other therapeutic agents, should not be used for the treatment of obesity or for weight loss. In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction. Larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given in association with sympathomimetic amines such as those used for their anorectic effects'.

Sources: Synthroid tablets prescribing information, Revised 2/2024.

What has FDA said about Armour Thyroid and other desiccated thyroid products?

FDA's page 'FDA's Actions to Address Unapproved Thyroid Medications' (FDA page content current as of August 6, 2026) says animal-derived thyroid medications 'are marketed as Armour Thyroid, NP Thyroid, Nature-Thyroid and Natural Thyroid, among other names', and states: 'Animal-derived thyroid medications are not FDA approved, yet an estimated 1.5 million patients received prescriptions for these medications from U.S. outpatient retail and mail-order pharmacies in 2024.'

FDA page

On its page 'FDA's Actions to Address Unapproved Thyroid Medications' (FDA page content current as of August 6, 2026), under the heading 'FDA plans to issue draft guidance', FDA states: 'FDA is currently applying its risk-based enforcement approach to unapproved animal-derived medications, and may take action.'

In a letter dated August 6, 2025 to manufacturers, importers and distributors of animal-derived thyroid products, FDA stated: 'ADT products require an approved biologics license application (BLA) under section 351 of the Public Health Service (PHS) Act in order to be legally marketed in the U.S. (42 U.S.C. 262(a)(1)). There are no FDA-approved BLAs for the ADT products currently on the market.'

FDA letter

Are compounded thyroid medicines FDA-approved?

Compounded levothyroxine and compounded liothyronine are different products from the tablets listed in the FDA status table.

FDA's page 'FDA's Actions to Address Unapproved Thyroid Medications' (FDA page content current as of August 6, 2026) states that 'these unapproved animal-derived thyroid medications are not eligible for compounding because these products are regulated as biological products under the Public Health Service Act.'

Compounded levothyroxine and compounded liothyronine are not FDA-approved. FDA states: 'Compounded drugs are not FDA-approved. This means that FDA does not verify the safety, effectiveness or quality of compounded drugs before they are marketed.' As of FDA's 503B list updated March 21, 2025, 'L-Triiodothyronine Sodium' and 'T3 Sodium Dilution' are in Category 3, 'Bulk Drug Substances Nominated Without Adequate Support', and this listing is not an FDA approval of any compounded product.

Do you need a prescription for thyroid medicine?

Each thyroid hormone product in the FDA status table is available only with a prescription (checked October 14, 2026).

Completing an intake form or making a payment does not guarantee a prescription.

Whether a product is appropriate for a specific person is decided by a licensed clinician using that product's prescribing information.

Regulatory status checked October 14, 2026. Sources: www.accessdata.fda.gov/…/021402s038lbl.pdf; www.accessdata.fda.gov/…/index.cfm; www.fda.gov/…/fdas-actions-address-unapproved-thyroid-medications; www.fda.gov/…/download; www.fda.gov/…/compounding-and-fda-questions-and-answers; www.fda.gov/…/download; www.fda.gov/…/download

How much does thyroid hormone therapy cost?

We do not list prices for thyroid hormone therapy on this page yet. Each pharmacy and provider sets its own prices.

What you pay can depend on the product prescribed, the visits and blood tests involved, and whether insurance applies.

A pharmacy can quote the medicine's price with and without insurance. What a provider's visit price includes, such as blood tests, varies by provider.

Compounded levothyroxine and compounded liothyronine are not FDA-approved. FDA states: 'Compounded drugs are not FDA-approved. This means that FDA does not verify the safety, effectiveness or quality of compounded drugs before they are marketed.' As of FDA's 503B list updated March 21, 2025, 'L-Triiodothyronine Sodium' and 'T3 Sodium Dilution' are in Category 3, 'Bulk Drug Substances Nominated Without Adequate Support', and this listing is not an FDA approval of any compounded product.

Does insurance cover levothyroxine?

Coverage varies by plan and can depend on the reason it is prescribed. Confirm coverage, and any prior authorization, with your insurer.

Questions to ask your insurer include whether the product your clinician prescribes is covered, whether prior authorization is needed, and whether the blood tests your clinician orders are covered.

What should I ask my doctor before starting thyroid hormone therapy?

These questions draw on thyroid association guidelines and on the Synthroid prescribing information. They are prompts for a conversation with a licensed clinician, and they do not show whether thyroid hormone therapy is appropriate for you.

1. Which blood tests showed that I have hypothyroidism, and were both TSH and free T4 measured?
2. My TSH is raised but my free T4 is normal: what would treatment be expected to change for me, given that trials in older adults found no improvement in symptoms?
3. How long after a dose change will my TSH be rechecked, and how often will it be checked once my dose is stable?
4. How should I time my tablet around breakfast and around iron, calcium or antacids?
5. Should I stop biotin supplements before my thyroid blood tests, and for how long?
6. Does my age, or any heart condition I have, change the dose I would start on?
7. If I am pregnant or planning a pregnancy, how will my dose and my blood tests change?
8. Which kind of thyroid hormone would I be taking: levothyroxine, liothyronine or desiccated thyroid?