S tier · elite
For hypothyroidism replacement the evidence is definitive and long-established — levothyroxine is a first-line, guideline-standard therapy with unambiguous biochemical and clinical efficacy and one of the largest prescribing bases of any medicine.
levothyroxine
Levothyroxine is synthetic T4, approved as replacement therapy in primary, secondary and tertiary hypothyroidism and for TSH suppression in thyrotropin-dependent well-differentiated thyroid cancer. Its efficacy for restoring euthyroidism in deficiency is not seriously contested; its label nonetheless carries a boxed warning against use for obesity or weight loss.
the explanation
Levothyroxine replaces the hormone an underactive thyroid cannot make, and it works reliably for that. The FDA's strongest warning on its label says it must not be used for weight loss, because in people with normal thyroid function it does not help and larger amounts can be dangerous.
regulatory status
FDA-approved with BOXED WARNING
Approved as replacement therapy in primary, secondary and tertiary hypothyroidism in adults and paediatric patients including neonates, and for pituitary TSH suppression as an adjunct to surgery and radioiodine in thyrotropin-dependent well-differentiated thyroid cancer. The label carries a boxed warning against use for obesity or weight loss.
how it works · proposed mechanism
Levothyroxine is exogenous thyroxine, a prohormone converted peripherally to the active hormone T3.
Deiodinase conversion
Type 1 and type 2 iodothyronine deiodinases remove an outer-ring iodine from T4 to generate T3. Because this step is tissue-regulated, circulating T4 acts as a buffered reservoir rather than a direct signal.
Nuclear receptor transcription
The resulting T3 binds thyroid hormone receptors acting as ligand-dependent transcription factors. Downstream effects include increased basal metabolic rate, protein turnover, and cardiac and neurological development.
Long half-life and stability
With a 6-7 day half-life, levothyroxine produces a flat serum profile in which day-to-day variation is smoothed out. This is the pharmacological reason T4 rather than T3 is the standard replacement agent.
what’s reported
evidence shape
5 sourcesThe composition of the cited literature by study type. This describes the shape of the evidence, not its quality — and it does not by itself set the grade.
⚠ the catch
The single most important sentence on the levothyroxine label is a boxed warning: thyroid hormones should not be used for the treatment of obesity or for weight loss, and doses beyond normal hormonal requirements may produce serious or life-threatening toxicity. The S grade here is for correcting a hormone deficiency, and it transfers not at all to the fat-loss use these compounds are sold for — in a euthyroid person, replacement doses do nothing for weight and supraphysiological doses are a documented toxicity hazard.
key published findings
- Regulatory (boxed warning, verbatim): 'Thyroid hormones, including SYNTHROID, should not be used for the treatment of obesity or for weight loss,' and doses beyond normal hormonal requirements 'may produce serious or life-threatening manifestations of toxicity.'
- Regulatory (label, pharmacokinetics): levothyroxine's elimination half-life is 6-7 days in euthyroid subjects, shortening to 3-4 days in hyperthyroidism and lengthening to 9-10 days in hypothyroidism — evidence that clearance is itself thyroid-status dependent.
- Regulatory (label, indications): approved across primary, secondary and tertiary hypothyroidism in adults and paediatric patients including neonates, plus TSH suppression in thyrotropin-dependent well-differentiated thyroid cancer.
- Human trial (randomised, double-blind, placebo-controlled): in the 141-subject LEVOLIO trial, adding liothyronine to levothyroxine produced no significant difference in SHBG, other tissue markers or quality of life versus levothyroxine plus placebo over 24 weeks — supporting T4 monotherapy as the reference standard.
- Regulatory (label, chemistry): levothyroxine sodium is the L-3,3',5,5'-tetraiodothyronine sodium salt, with the anhydrous sodium salt having a molecular weight of 798.86 and the free acid 776.87.
limitations of the evidence
- A minority of adequately replaced patients report persistent symptoms despite normalised TSH, and randomised attempts to address this with added liothyronine have largely failed, so the mechanism of residual symptoms is unresolved.
- Much of the efficacy evidence predates modern trial standards and rests on biochemical normalisation rather than randomised patient-reported outcomes.
- There is no randomised evidence for levothyroxine in euthyroid people for weight or body composition, because that use is warned against on the label rather than studied.
documented safety signals
- BOXED WARNING: thyroid hormones must not be used for obesity or weight loss; in euthyroid patients replacement-range doses are ineffective for weight reduction and larger doses risk serious or life-threatening toxicity.
- The boxed warning specifically flags that toxicity risk is greater when thyroid hormone is given with sympathomimetic amines such as those used for anorectic effect.
- Over-replacement causes iatrogenic thyrotoxicosis: tachycardia, palpitations, arrhythmias including atrial fibrillation, angina, tremor, heat intolerance, anxiety and insomnia.
- Excess thyroid hormone accelerates bone resorption and is associated with decreased bone mineral density, particularly in postmenopausal women and with long-term TSH-suppressive exposure.
- Cardiac risk is concentrated in the elderly and in patients with underlying coronary artery disease, in whom rapid changes may precipitate ischaemia.
- Myxoedema coma and adrenal crisis are risks when thyroid hormone is manipulated without addressing concurrent adrenal insufficiency.
- Narrow therapeutic index means absorption interactions matter: calcium, iron, proton pump inhibitors, bile acid sequestrants and numerous other agents alter exposure.
- Effects on anticoagulant response, digitalis glycosides and antidiabetic requirements are labelled and require monitoring.
- Because half-life is 6-7 days, over-exposure resolves slowly, and the consequences of a supraphysiological amount persist for weeks.
identity
| full name | Levothyroxine sodium (thyroxine, T4) |
| category | Thyroid & Stimulant |
| modality | small molecule |
| formula | C15H11I4NO4 |
| molar mass | 776.87 g/mol |
| cas | 51-48-9 |
| half-life | 6-7 days (3-4 days in hyperthyroidism; 9-10 days in hypothyroidism) |
laboratory handling
| storage | Marketed tablets are stored at controlled room temperature and protected from light and moisture per the approved label; levothyroxine potency is sensitive to heat, light and humidity, which is why formulation and storage are tightly specified. |
| solubility | Levothyroxine free acid is very poorly water-soluble; the marketed sodium salt is more soluble, and absorption is pH-dependent and readily disrupted by chelating agents in the gut. |
| co-studied with | The FDA boxed warning explicitly addresses combination: it states that toxicity risk is greater when thyroid hormone is administered together with sympathomimetic amines, including those used for their anorectic effect. Documented as a labelled hazard, not as a protocol. |
Handling information describes laboratory practice for a research reagent. It is not a preparation guide for use in humans.
the receipts
5 citedUS FDA Drugs@FDA · 2024 · official
US FDA Drugs@FDA · 2017 · official
StatPearls, NCBI Bookshelf · 2024 · review
DailyMed / US FDA · 2023 · review
European Journal of Endocrinology · 2024 · randomized
others in Thyroid & Stimulant