Pharmacology · Class
Antithyroid agents !
Medicines that lower thyroid hormone production in hyperthyroidism: thionamides, iodide and radioactive iodine.
Updated 2026-11-01
What they are
Drugs for hyperthyroidism such as Graves disease and toxic nodular goitre. Thionamides (methimazole, carbimazole, propylthiouracil) block hormone synthesis; iodide in high amounts briefly blocks hormone release and shrinks the gland before surgery; radioactive iodine (131I) destroys thyroid tissue. Beta-blockers control symptoms but do not treat the gland.
Mechanism of action
Thionamides inhibit thyroid peroxidase, which oxidises iodide, attaches it to tyrosines of thyroglobulin (organification) and couples iodotyrosines into T4 and T3. They do not block iodide uptake or the release of stored hormone, so the effect appears only after weeks. Propylthiouracil also blocks peripheral conversion of T4 to T3. High iodide inhibits organification and hormone release; 131I is taken up by the sodium-iodide symporter and destroys the cells by radiation.
Target: Thyroid peroxidase (thionamides); sodium-iodide symporter and the thyroid gland (iodide, 131I)
Drugs in this class
- MethimazoleThe first-choice tablet for an overactive thyroid. It blocks the enzyme that builds thyroid hormone; rarely it wipes…Thyroid peroxidase (TPO)
- PropylthiouracilAn antithyroid tablet kept for the first trimester of pregnancy, thyroid storm or methimazole intolerance, because it…Thyroid peroxidase (TPO)
- Potassium iodideStable iodide that floods the thyroid. It protects the gland in a radiation emergency and briefly slows hormone…Sodium-iodide symporter (NIS) of the thyroid
- Sodium iodide (131I)Radioactive iodine swallowed as a capsule or liquid. The thyroid takes it up and its radiation destroys overactive or…Sodium-iodide symporter (NIS) of the thyroid
- CarbimazoleA thionamide prodrug: the body turns it into methimazole. Widely used for an overactive thyroid in the UK and other…Thyroid peroxidase (TPO), through methimazole
Others in the class
- Iodine with potassium iodide (Lugol solution)
- Potassium perchlorate
Test yourself
Think of the answer, then tap the question to see it.
Which enzyme do thionamides inhibit, and which steps of hormone synthesis does it catalyse?
Thyroid peroxidase: oxidation and organification of iodide on thyroglobulin and coupling of iodotyrosines into T4 and T3.
A patient on methimazole has fever and a sore throat. What is the first step?
Stop the drug and check a complete blood count urgently to rule out agranulocytosis.
Why does a thionamide take weeks to lower thyroid hormone levels?
It blocks new synthesis but not the release of hormone already stored in thyroglobulin, which must be used up first.
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - METHIMAZOLE tablet (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - PROPYLTHIOURACIL tablet (DailyMed (U.S. National Library of Medicine), 2026); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 38 (McGraw Hill, 2025); eEML - Electronic Essential Medicines List (World Health Organization, 2025).
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Representative model, for educational purposes only.