Synthetic thyroid hormone (T4) that replaces what an underactive thyroid cannot make, restoring energy, weight and metabolism; the dose is set by regular TSH blood tests.
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What it is used for
Hypothyroidism from any cause (autoimmune, after surgery or radioiodine, congenital)
Suppressing TSH after thyroid cancer
Subclinical hypothyroidism when TSH is high, symptomatic, or in pregnancy
How it works in the body
A synthetic copy of thyroxine (T4), the hormone the thyroid normally releases. Tissues convert it to the active T3, which enters cell nuclei and switches on genes controlling metabolic rate, heat production, heart rate, gut motility, growth and brain development. Replacement restores the pituitary's TSH to normal, which is how the dose is tuned.
Common side effects
Few at the right dose; at too high a dose: palpitations, weight loss, sweating, anxiety, insomnia, diarrhoea (symptoms of hyperthyroidism)
Temporary hair loss in the first months
Serious: seek help
Angina or atrial fibrillation if started too fast in the elderly or those with heart disease (start at 25 micrograms)
Bone loss and atrial fibrillation with long-term over-replacement
Adrenal crisis if given before untreated adrenal insufficiency is corrected
Cautions and interactions
Absorption falls with calcium, iron, antacids, proton pump inhibitors, soya and coffee: separate by 4 hours
Dose often needs to rise in pregnancy (increase by 25-50% and check TSH promptly), and with oestrogen, enzyme inducers
Stay on the same brand where possible
Do not use for weight loss in people with normal thyroid function
Interactions
Check interactions
Taking Levothyroxine with other medicines? Add them to the checker to see what the official sources say about each pair: the mechanism, general management and monitoring, never a bare "safe".
Interactions documented in the official sources cited on each card. The list covers the medicines represented on this site and is not exhaustive; how the interaction data is compiled.
Medicines that share the class of Levothyroxine. They are separate medicines with their own licences, doses and interactions, not alternative names for Levothyroxine.
Medicines often discussed alongside Levothyroxine: used together, compared with it, or acting on the same problem by a different route.
Educational content. Anatomy Nexus provides medical education and does not replace professional medical advice, diagnosis, treatment, prescribing or pharmacist review. Doses, choices and monitoring are decided by a prescriber for an individual; never start, stop or change a medicine on the basis of this page.
