Levothyroxine

T4 replacement

The US Synthroid label covers primary, secondary and tertiary hypothyroidism, including congenital and acquired disease. It also covers TSH suppression in selected differentiated thyroid cancer. It is not approved as a general energy or weight-loss drug.³⁹³

Record	What it means
Levothyroxine	The free-acid identity. PubChem CID 5819 and KEGG D08125 refer to this form.
Levothyroxine sodium	A sodium salt with its own chemical record. The anhydrous form is listed separately from hydrates.
Levothyroxine sodium hydrate	The ingredient named in the Japanese Thyradin S tablet label. Hydrate wording should stay product specific.
Tablet, capsule and oral solution	Different dosage forms with different labels. A result or age range for one should not be copied to another.

Overt and mild disease

A replacement treatment can be established for hormone deficiency while a trial in mild disease still finds little benefit.

Overt and congenital hypothyroidism

Levothyroxine replaces T4 when the thyroid does not supply enough hormone. Evidence from mild subclinical disease does not erase that role. Central hypothyroidism also needs a different monitoring method because TSH alone is not reliable.³⁹³

737 people

TRUST enrolled adults aged 65 or older with persistent subclinical hypothyroidism and normal free T4. At one year, levothyroxine lowered TSH but did not improve the primary symptom or tiredness scores compared with placebo.⁴⁰⁵

The TRUST population

The group had subclinical disease and low average symptom burden. The trial does not answer overt or congenital hypothyroidism, younger populations or pregnancy. A Korean guideline reaches a cautious treatment recommendation for adults aged 70 or older with subclinical disease, but that guideline summary is not a new treatment trial.^405,406

Pregnancy changes the question

Established replacement, mild subclinical disease and isolated hypothyroxinemia describe more than one clinical population.

Established hypothyroidism

The current US label says levothyroxine replacement should not be discontinued during pregnancy. Untreated maternal hypothyroidism carries risks for the pregnant person and fetus. This is different from starting treatment for a mild screening result.³⁹³

677 and 526

One trial report included 677 women with subclinical hypothyroidism and 526 with hypothyroxinemia. Treatment began at mean gestational ages of 16.7 and 17.8 weeks. It did not improve the reported child-IQ or pregnancy and neonatal outcomes. The result covers the two reported cohorts and does not answer earlier treatment for overt disease.⁴⁰⁸

The 2026 guideline uses timing and severity

The 2026 American Thyroid Association guideline separates overt disease from mild disease. It says first-trimester treatment for mild subclinical disease may be considered, while mild subclinical hypothyroidism first found after the first trimester and isolated hypothyroxinemia should not be offered treatment. Diagnosis and timing determine which one of these guideline recommendations applies.⁴⁰⁷

Formulation and absorption

The same active hormone can behave differently across products, food conditions and laboratory tests.

Product form	Boundary
Synthroid tablet	A levothyroxine sodium tablet label with its own indications, warnings and monitoring language.
Tirosint capsule	The cited US label covers adults and children aged 6 years and older.
Tirosint-SOL oral solution	The cited US label includes neonates. That age scope cannot be copied to the capsule.

Switching products

UK guidance says generic prescribing is appropriate for most people. It also recognizes a minority who report symptoms after switching. The Yellow Card reports do not provide a rate or prove that switching caused every symptom.⁴⁰⁹

Food and coffee can change tablet absorption

Labels describe food and binding products as absorption concerns. A small study also found reduced tablet absorption with coffee.^393,410

Drug and test interactions: three types

European safety review records describe reduced tablet absorption with proton-pump inhibitors, increased clearance with St John’s wort and misleading thyroid-test results with biotin. Biotin can distort an assay without changing levothyroxine exposure.^411,412

Symptoms can outlast a normal TSH

Persistent symptoms are real, but the trials do not point to one universal replacement strategy.

TSH by diagnosis

TSH usually guides treatment in primary hypothyroidism. The Synthroid label says TSH alone is unreliable in secondary or tertiary hypothyroidism and uses free T4 instead. A normal TSH should not be interpreted without the disease context.³⁹³

46 adults

A four-month trial compared levothyroxine plus liothyronine with levothyroxine alone. It found no benefit in weight, lipids, symptoms, cognition or wellbeing. One small fixed-regimen trial cannot settle every patient’s experience.⁴¹⁴

A three-way crossover had a mixed signal

A 75-person crossover compared levothyroxine, levothyroxine plus liothyronine and desiccated thyroid. Whole-group outcomes were similar. A post hoc analysis found preference and response signals in the most symptomatic third, which is a reason for further study rather than a universal switch rule.⁴¹⁵

Normal thyroid tests are another population

A small crossover trial enrolled 25 symptomatic adults with thyroid tests in range, of whom 22 completed, plus 19 healthy controls who all completed. Thyroxine did not improve cognitive or psychological outcomes in the symptomatic group.⁴¹⁶

Safety, recalls and US status

The pages below cover overreplacement, one discontinued brand and one recalled lot.

Overreplacement and weight loss

The US label warns against using thyroid hormone for obesity or weight loss. Excess exposure can cause a fast or irregular heartbeat, chest pain, tremor, insomnia, sweating, diarrhea, weakness and weight loss. Long-term overreplacement can also harm bone.³⁹³

The US Euthyrox action was product specific

FDA withdrew approval of Euthyrox NDA 021292 on August 5, 2026 after the applicant said it was no longer marketed. Other approved synthetic T4 products remained in FDA records.^417,403,418

Record	Scope
FDA synthetic levothyroxine records	Historical records, including 2026 rows, show Class II subpotency events. Enforcement rows are not patient counts or a defect rate.
August 2026 Class I recall	One lot of a porcine T4-and-T3 thyroid product, not synthetic levothyroxine.
Kazakhstan, November 2025	Three named Levotiron lots failed a packaging parameter. The record did not show a potency or sterility failure.
India, September 2022	One named 50 microgram levothyroxine batch appeared in an assay alert. It does not establish a current class-wide defect.

Regional studies

The records cover a surrogate nodule result, a small pediatric formulation review and a rat toxicity model. Their results apply to those specific questions and formulations.

A Chinese benign-nodule abstract

A 2015 publisher abstract reports 124 people randomized 62 to 62 to low-dose levothyroxine or observation for 12 months. It reports changes in nodule size, thyroid volume and laboratory measures. The abstract leaves blinding, allocation concealment, attrition, clinical outcomes and detailed harms unclear.⁴²²

A small Japanese tablet study

A retrospective study reviewed 13 children younger than three who received tablets directly. Table 1 lists 12 previously receiving powder and one with no prior treatment. Nine had serial pre-and-post laboratory data; a poor-adherence case was excluded from mean-and-variance comparisons, and one central case contributed only free T4. The paper reports no significant change in the analyzed thyroid measures, but it does not state one final denominator for every measure.⁴²³

The Russian paper was an animal model

A Russian experiment began with 44 Wistar rats and exposed the remaining 34 to levothyroxine for two weeks to model hyperthyroidism. It had no randomized parallel placebo group. It is not human treatment evidence and does not support a selective weight-loss claim.⁴²⁴

Common questions

Food, coffee, symptoms, test results, pregnancy and product differences.

01 Can I take levothyroxine with food, coffee, calcium, or iron?

Coffee taken with levothyroxine tablets can reduce short-term absorption. In a small study, coffee taken at the same time lowered T4 absorption measures, while coffee 60 minutes later resembled water. Food, calcium and iron can also interfere, and exact timing depends on the named product and other medicines or supplements.^393,410,411

02 Are Synthroid, generic levothyroxine, Tirosint, and other formulations interchangeable?

They contain levothyroxine, but tablets, capsules and oral solution have separate product records and are not one universal formulation. UK regulators say generic prescribing is appropriate for most people, while a minority report symptoms after switching. Symptoms or changed test results after a switch need product-specific review.^{393,401,402,409,413}

03 Should T3, liothyronine, or desiccated thyroid be added instead?

Not as a rule for everyone. Whole-group results in randomized T4, T4-plus-T3 and desiccated-thyroid comparisons were often similar, while a selected unblinded symptomatic group improved on T3 and had more withdrawals on T3. A small postsurgical feasibility study with 12 analyzed participants had nonsignificant clinical differences.^{414,415,425,426,427,428}

04 How do TSH, free T4, and biotin affect monitoring?

TSH is the main laboratory guide in primary hypothyroidism, but the current label says it is unreliable by itself in secondary or tertiary hypothyroidism and uses free T4 instead. Biotin can make some thyroid tests look falsely high or low without changing levothyroxine exposure. The lab and clinician need the supplement and product context to interpret a result.^393,402,411

05 What side effects can levothyroxine cause?

The label says most adverse effects reflect too much thyroid hormone, including a fast or irregular heartbeat, chest pain, tremor, insomnia, sweating, diarrhea, muscle weakness and weight loss. Older people and those with heart disease have a higher cardiac-risk warning, and sustained overreplacement can harm bone. Rash and rare early partial hair loss are also listed.³⁹³

06 Has levothyroxine been recalled or in shortage?

Dated FDA records include synthetic-levothyroxine Class II subpotency recalls, including 2026, while the separate 2026 Class I thyroid-tablet event involved a porcine T4-and-T3 product. FDA withdrew approval of the discontinued US Euthyrox NDA in August 2026 because it was no longer marketed, while other synthetic T4 products remain approved. Those cited FDA records describe specific recalls and a discontinued product; they do not answer whether there is a current class-wide levothyroxine shortage.^{419,404,417,403}

07 Does levothyroxine cause weight gain or weight loss?

Levothyroxine is not a weight-loss medicine. The label says replacement-range amounts are ineffective for weight reduction in people with normal thyroid function, while excessive exposure can cause toxic weight loss and other hyperthyroid effects. The sources behind this chapter do not provide a guaranteed direction or amount of weight change for an individual.³⁹³

08 What is levothyroxine for?

Levothyroxine replaces T4 in people with primary, secondary, tertiary, congenital, or acquired hypothyroidism. The US Synthroid label also covers TSH suppression in selected differentiated thyroid cancer. It is not approved as a general energy or weight-loss drug.^393,394

09 Can I take levothyroxine during pregnancy or breastfeeding?

Levothyroxine replacement is used in pregnancy; the current label says not to discontinue it, and untreated hypothyroidism carries maternal and fetal risks. The 2026 ATA guideline treats mild cases differently by diagnosis and timing: first-trimester mild subclinical disease may be considered, while mild subclinical hypothyroidism found after the first trimester and isolated hypothyroxinemia should not be offered treatment; overt disease is separate. Levothyroxine is present in human milk, but the label reports no adverse effects in breastfed infants from published studies.^393,407

10 Can levothyroxine be used for weight loss?

No. The boxed warning says thyroid hormone should not be used for obesity or weight loss, and large amounts can cause serious or life-threatening toxicity, especially with sympathomimetic drugs. Any weight loss from excess thyroid hormone reflects overexposure rather than selective fat loss.³⁹³

11 What if I miss a dose?

Follow the missed-dose instructions for your exact product or ask your pharmacist or prescriber. A catch-up plan is not the same for every product and patient.^393,401,402

12 Does levothyroxine help mild or subclinical hypothyroidism?

Not automatically. In TRUST, 737 adults aged 65 or older with persistent subclinical hypothyroidism had lower TSH but no benefit on the primary symptom or tiredness scores at one year. That result does not answer overt or congenital deficiency, younger populations or pregnancy, where the 2026 ATA guideline separates severity and timing.^405,393,407

13 How long does levothyroxine take to work?

The current US labels say the peak effect of a given amount may take four to six weeks and that symptom improvement may take several weeks. That is a product-label timeline, not a promise that every symptom will resolve or a reason to change the amount without clinical review.^393,402

14 Does levothyroxine cause hair loss?

The current US labels say partial hair loss may occur rarely during the first few months and is usually temporary.^393,402

15 Why do I still have symptoms when my TSH is in range?

A normal TSH usually guides primary hypothyroidism, but the Synthroid label says TSH alone is not reliable in secondary or tertiary hypothyroidism. Trials of T3 or desiccated thyroid did not show one answer for everyone, although selected symptomatic groups showed signals. Persistent symptoms need reassessment rather than assuming the levothyroxine amount is the only cause.^{393,416,414,415}