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Thyroid Hormone

Levothyroxine

Brand names: Eltroxin, Euthyrox, Synthroid

Used in: Thyroid Disorders

Levothyroxine is synthetic thyroxine (T4) used as replacement therapy in 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

Levothyroxine is converted in the tissues to the active hormone tri-iodothyronine (T3), restoring normal metabolic activity in hypothyroid patients.

Prescribing in practice

  • Start low and titrate gradually in older patients or those with ischaemic heart disease, to avoid precipitating angina or arrhythmia.
  • Absorption is affected by food, calcium, iron and some other drugs — take on an empty stomach and separate from these.
  • Dose requirements rise in pregnancy; review thyroid function early and adjust.

Monitoring

Adjust dose to TSH (and free T4), rechecking after about 6–8 weeks following any change, then periodically once stable.

Counselling the patient

  • Take it on an empty stomach (e.g. 30–60 minutes before breakfast) at the same time daily.
  • Separate it from calcium or iron supplements by several hours.
  • Do not stop it — it is lifelong replacement; tell your clinician if you become pregnant.

Evidence & guidelines

Levothyroxine is the standard treatment for primary hypothyroidism, with TSH-guided titration to the reference range.

Reference: BTA/BAET Guidelines 2019; ATA Hypothyroidism Guidelines 2014; 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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