Thyroid agents · Endocrine
Levothyroxine
L-thyroxine · T4 · Synthroid® · Eltroxin® · Euthyrox®(brand names vary by market)
A hormone with a six-week feedback loop and a long list of things that block it.
Also: capsule, injection
Mechanism
Synthetic T4, converted peripherally to the active T3 at a rate the body controls. Replacing the prohormone lets physiology set the pace.
Indications
- Hypothyroidism
- Suppression therapy in thyroid cancer
- Myxoedema coma (intravenous)
Formulations
Dosing concepts
Starting
Full replacement is reasonable in a young, healthy patient; in an older patient or anyone with coronary disease it is started low and increased slowly, because raising metabolic demand on a stenosed coronary artery can precipitate angina.
Timing of the check
TSH is rechecked no sooner than six weeks after a change. Checking earlier measures the previous dose and invites a wrong adjustment.
Absorption
Taken fasting, and separated from calcium, iron, and proton pump inhibitors by several hours. Most 'dose failures' are absorption failures.
Concepts, not prescribing instructions. Practice numbers live in current references and local protocols.
Pharmacokinetics
- Bioavailability
- 40 to 80%, markedly reduced by food and binding agents
- Half-life
- ~7 days
- Elimination
- hepatic: Deiodination in peripheral tissues; the long half-life is why steady state takes about six weeks
- Protein binding
- >99%
Adverse effects
- Symptoms of over-replacement: palpitations, tremor, anxiety, heat intolerance, weight losscommon
- Atrial fibrillationseriousParticularly in older adults with a suppressed TSH.
- Reduced bone density with long-term over-replacementserious
- Angina in coronary diseaseserious
Contraindications & precautions
Contraindications
- Untreated adrenal insufficiency
- Untreated thyrotoxicosis
- Acute myocardial infarction
Precautions
- Coronary artery disease
- Older adults
- Pregnancy, where requirements rise early and substantially
Interactions
No curated interaction entries yet for this agent.
Monitoring
TSH
The primary endpoint · 6 to 8 weeks after any change, then every 6 to 12 months once stable
Free T4 where TSH is unreliable
Central hypothyroidism, recent thyrotoxicosis, pregnancy
Symptoms and pulse
Over-replacement often shows here before the number moves
Counselling points
- Take it on an empty stomach, at least 30 to 60 minutes before food, at the same time each day.
- Keep calcium, iron and antacids at least four hours away from it.
- Do not switch brands casually; if a brand changes, we recheck your levels.
- Tell us straight away if you become pregnant: the dose usually needs to go up quickly.
- It takes weeks to feel a change. That is normal.
Clinical pearls
- A rising TSH on an unchanged dose is an absorption question first: a new proton pump inhibitor, calcium supplement or iron tablet explains most of them.
- In pregnancy, requirements rise by roughly a quarter to a third and they rise early. This is one of the few dose changes made before a blood test rather than after it.
- Levothyroxine given to an undiagnosed adrenally insufficient patient can precipitate an adrenal crisis: cortisol before thyroxine when both are in question.
References & review
- Levothyroxine product monograph (consult the current version for your market) (monograph)
- Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. (reference-work)
Last reviewed 2026-08-16 · jurisdiction: global · drug information changes; verify against current references before practice use.