Liothyronine: the faster-acting thyroid hormone

Liothyronine sodium is the synthetic form of triiodothyronine. Most thyroid replacement in the UK uses levothyroxine (T4), which the body converts into T3. The emc information describes liothyronine as working similarly but with a crucial difference: its effects begin within a few hours and disappear within twenty-four to forty-eight hours of stopping treatment. That speed is why it features in urgent situations.

This guide uses the emc SmPC for liothyronine tablets because a dedicated NHS patient page was not available in the A to Z list that was checked.

The emc information lists myxoedema coma, severe chronic thyroid deficiency and hypothyroidism. It can also serve as an add-on to carbimazole during the treatment of thyrotoxicosis to prevent subclinical hypothyroidism. Who should be given it, and when, is a specialist decision; endocrinology teams manage most use because evidence for combined T3 and T4 treatment in everyday hypothyroidism is limited and carries risks if overtreated.

The SmPC describes tablets taken in divided doses through the day for adults, starting low and increasing after about a week, with much smaller starting amounts in children and older adults. Because the effect is short, dose timing matters more than with levothyroxine. Always follow the exact schedule on your own label.

Established heart disease is the major issue. The SmPC says liothyronine is contraindicated in established myocardial ischaemia, and a baseline ECG is recommended before treatment. Severe hypothyroidism can mask adrenal insufficiency, so corticosteroid cover may be needed. TSH is monitored because over-treatment can cause atrial fibrillation and bone thinning. Diabetes medicine needs may rise because thyroid hormone affects glucose control.

Because liothyronine acts quickly, symptoms of too much hormone can appear within hours: a racing or irregular heartbeat, tremor, sweating, anxiety or difficulty sleeping. The SmPC lists over-treatment complications such as atrial fibrillation, so report palpitations or chest pain rather than waiting for your next appointment. Keep every monitoring appointment, take tablets at the times on your label, and keep a list of every medicine you use to show to clinicians. If you are unwell and cannot swallow tablets, contact your prescriber or NHS 111 because stopping a thyroid hormone abruptly can also cause problems.

Liothyronine can strengthen the effect of anticoagulants, may raise phenytoin levels and can alter blood sugar control. As with other thyroid hormones, tell any prescriber what you take, including supplements, before starting something new, and tell the person ordering thyroid tests which thyroid medicine you use.

The SmPC says safety in pregnancy is undocumented, so use is decided case by case with a specialist. Liothyronine passes into breast milk at low concentrations and may interfere with newborn screening tests, so tell the midwife or health visitor.

Liothyronine is prescription only. The emc lists tablets and capsules of several strengths from different manufacturers, as well as an oral solution and an injection used in hospital. Suspected side effects can be reported through the Yellow Card scheme.