Thyroid agents

Replace thyroid hormone, or block its synthesis. Both directions are titrated against TSH, and both are slow.

Members

LevothyroxineA hormone with a six-week feedback loop and a long list of things that block it.Methimazole (thiamazole)Blocks hormone synthesis, and can silently stop the bone marrow.

Also in this class: Liothyronine · Propylthiouracil. Full pages arrive as the library grows.

Compare the members

Drug	Direction	Timing	Note
Levothyroxine	Replace	Fasting, 30 to 60 min before food	Separate from calcium and iron by 4 h
Methimazole (thiamazole)	Block synthesis	Once daily	Agranulocytosis warning is mandatory

Levothyroxine

Direction: Replace
Timing: Fasting, 30 to 60 min before food
Note: Separate from calcium and iron by 4 h

Methimazole (thiamazole)

Direction: Block synthesis
Timing: Once daily
Note: Agranulocytosis warning is mandatory

Shared across the class

Indications: Hypothyroidism (replacement) · Hyperthyroidism (antithyroid)
Adverse effects: Over- or under-treatment symptoms · Rash (antithyroid)
Contraindications: Untreated adrenal insufficiency (levothyroxine) · Prior agranulocytosis on an antithyroid drug
Monitoring: TSH no sooner than 6 weeks after any dose change: earlier is measuring the old dose · Free T4 when TSH is unreliable (pituitary disease, recent thyrotoxicosis) · Antithyroid drugs: warn about sore throat and fever, and check the white count if they occur

What makes each agent different

Levothyroxine is T4, a prodrug the body converts at its own pace; that is a feature, and it is why liothyronine (T3) is rarely needed and easy to overshoot with.
Methimazole is once daily and preferred except in the first trimester and thyroid storm, where propylthiouracil takes over for teratogenicity and conversion-blocking reasons respectively.

Levothyroxine absorption is wrecked by calcium, iron, and proton pump inhibitors. A rising TSH on an unchanged dose is a co-administration question before it is a dose question.

Six weeks is not bureaucratic caution: it is roughly three half-lives of T4, and a TSH checked at two weeks will send you chasing a number that has not finished moving.

Last reviewed 2026-08-16 · Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed.