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Thyroid hormone (T4)

Levothyroxine sodium

Brand names: Eltroxin, various generics

Used in: Thyroid Disorders

Levothyroxine (T4) is the standard replacement treatment for hypothyroidism.

Dosing — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses against the product SmPC and your local formulary before prescribing.

US labelling (FDA)

Reference — US labelling, may differ from UK

Administer once daily, preferably on an empty stomach, one-half to one hour before breakfast with a full glass of water. (2.1) Administer at least 4 hours before or after drugs that are known to interfere with absorption. (2.1) Evaluate the need for dose adjustments when regularly administering within one hour of certain foods that may affect absorption. (2.1) Advise patients to stop biotin and biotin-containing supplements at least 2 days before assessing TSH and/or T4 levels. (2.2) Starting dose depends on a variety of factors, including age, body weight, cardiovascular status, and concomitant medications. Peak therapeutic effect may not be attained for 4 to 6 weeks. (2.2) See full …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2026-03-24. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Clinical monograph

How it works

It is a synthetic form of thyroxine (T4) that the body converts to active T3, restoring normal thyroid hormone levels.

Prescribing in practice

  • Titrate the dose against TSH (and free T4), allowing several weeks between dose changes.
  • Absorption is reduced by food, calcium, iron and some other drugs — take it on an empty stomach, separated from these.
  • Start low and titrate cautiously in older people and in ischaemic heart disease (risk of provoking angina/arrhythmia); over-replacement risks atrial fibrillation and reduced bone density.

Monitoring

Monitor TSH (and free T4) periodically and after dose changes; review cardiac symptoms in at-risk patients.

Counselling the patient

  • Take it once daily on an empty stomach (e.g. 30–60 minutes before breakfast), away from calcium or iron supplements.
  • Take it consistently and don't stop without advice.
  • Report palpitations.

Evidence & guidelines

The standard treatment for hypothyroidism (NICE NG145), titrated to TSH.

Reference: NICE NG145; British Thyroid Association; MHRA Drug Safety Update; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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