Thyroid Hormone Pregnancy: UK SPC (section 4.6): safety in pregnancy not established; weigh any possible risk of foetal abnormality against the risk to the foetus of untreated hypothyroidism (hypothyroidism should be treated in pregnancy). Combination of levothyroxine with anti-thyroid agents for hyperthyroidism is not indicated in pregnancy. Excreted in breast milk in low concentrations.

Levothyroxine

Brand names: Eltroxin, Euthyrox, Synthroid

Used in: Thyroid Disorders

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

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Initially 100 micrograms daily (adults under 50 years without cardiac disease)

Route: Oral

Frequency: Once daily, preferably before breakfast or the first meal of the day

Treatment of hypothyroidism. Adults under 50 years without cardiac disease: initially 100 micrograms daily; adjust by 50 micrograms at 3-4 week intervals until normal metabolism is steadily maintained; final daily dose usually 100-200 micrograms. Patients over 50 years without cardiac disease: initially do not exceed 50 micrograms daily; increase by 50 micrograms every 3-4 weeks; final daily dose 50-200 micrograms. Patients over 50 years with cardiac disease: 25 micrograms daily or 50 micrograms on alternate days; increase by 25 micrograms every 4 weeks; final daily dose 50-200 micrograms. Elderly: as for patients over 50 years. Introduce very gradually in patients over 50 and in long-standing hypothyroidism. If metabolism increases too rapidly (diarrhoea, nervousness, rapid pulse, insomnia, tremor, anginal pain) reduce or withhold the dose for 1-2 days then restart at a lower dose. Monitor serum TSH; a pre-therapy ECG is advised. Paediatric dosing is age-, weight- and indication-specific (congenital hypothyroidism, acquired hypothyroidism, juvenile myxoedema) using weight-based and body-surface-area-based regimens; children under 5 years should be given an approved oral levothyroxine solution rather than tablets - verify all paediatric doses against a children's formulary. Source product: Levothyroxine 100 micrograms Tablets (UK SPC).

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm 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.

Contraindications

Hypersensitivity to levothyroxine or any excipient
Thyrotoxicosis
Adrenal gland disorder or adrenal insufficiency
Do not initiate treatment in acute myocardial infarction, acute myocarditis, or acute pancarditis
Combination therapy of hyperthyroidism with levothyroxine plus anti-thyroid agents is not indicated in pregnancy

Side effects

Cardiac: angina pectoris, arrhythmia, palpitations, tachycardia
Nervous system / psychiatric: tremor, restlessness, agitation, insomnia, headache
Gastrointestinal: diarrhoea, vomiting
Skin: hyperhidrosis, alopecia, rash, pruritus, flushing
Other: weight decreased, pyrexia, malaise, oedema; thyrotoxic crisis with severe over-replacement. Side-effects usually indicate excessive dosage and resolve on dose reduction or brief withdrawal

Interactions

Phosphate binders (calcium carbonate, ferrous sulfate, sevelamer, lanthanum) may bind levothyroxine and reduce absorption - give at least 4 hours apart (US labelling section 7)
Bile acid sequestrants (colesevelam, cholestyramine, colestipol) and ion-exchange resins (e.g. Kayexalate) decrease levothyroxine absorption (US labelling section 7)
Orlistat - monitor thyroid function during concomitant use (US labelling section 7)
Antidiabetic therapy (insulin, metformin): thyroid replacement may increase insulin or anti-diabetic dose requirements (UK SPC section 4.4)

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. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Drugs

Levothyroxine (Elderly) · Thyroid Hormone

Pathways