The thyroid gland is a major endocrine organ that produces thyroxine (T4), triiodothyronine (T3), and calcitonin, with T3 and T4 playing a central role in regulating metabolic rate. For more information on the thyroid gland, please see the related article The thyroid gland. Thyrotoxicosis describes the hypermetabolic state caused by elevated circulating free T3 and T4. Symptoms include tachycardia, diarrhoea, anxiety, muscle weakness and sweating. Hyperthyroidism refers specifically to hyperfunction of the thyroid gland and is classified as primary or secondary. Primary hyperthyroidism Caused by intrinsic thyroid pathology: Graves’ disease (diffuse thyroid hyperplasia) Toxic multinodular goitre Toxic adenoma Hyperfunctioning thyroid carcinoma Iodine‑induced hyperthyroidism Secondary hyperthyroidism Caused by pathology external to the thyroid gland: TSH‑secreting pituitary tumour This article outlines the pharmacology, therapeutics and key prescribing points for drugs used in the management of hyperthyroidism. Treatment options Treatment involves symptomatic management (e.g. beta‑blockers) and pharmacological or surgical suppression of thyroid hormone production. Common pharmacological treatments include radioiodine therapy, thioureylenes and iodide. Created in BioRender. Boucher, M. (2026) https://BioRender.com/1vhzmft Fig 1: Mechanism Of Action Of Drugs For Hyperthyroidism Radioiodine Therapy Radioiodine uses the radioactive isotope iodine‑131 (¹³¹I) to selectively damage thyroid follicular cells and reduce thyroid hormone production. Mechanism of action ¹³¹I is taken up by follicular cells via iodide transporters and incorporated into thyroglobulin. It emits both beta and gamma radiation. Gamma radiation passes through tissue with minimal effect, whereas beta particles are absorbed locally, causing follicular cell destruction and reduced hormone synthesis. Therapeutic considerations Administered orally as a single dose, typically 400–800 MBq Radioactive effects persist for ~2 months (half‑life ≈8 days) Clinical effect begins after 1–2 months, with maximal effect after a further 2 months >50% of patients develop hypothyroidism and require lifelong thyroid hormone replacement Contraindicated in pregnancy, breastfeeding and children Patients should: Avoid pregnancy for 6 months after treatment Avoid fathering children for 4 months after treatment No increased incidence of thyroid cancer has been demonstrated following therapeutic use Thioureylenes Thioureylenes contain a thiocarbamide (S–C–N) group and include carbimazole, methimazole and propylthiouracil (PTU). Their mechanism is not fully understood, but they inhibit iodination of thyroglobulin, reducing synthesis of T3 and T4. Propylthiouracil additionally inhibits peripheral conversion of T4 to T3. Although iodination is reduced by ~90% within 12 hours, clinical improvement takes several weeks due to large intrathyroidal hormone stores and the long half‑life of T4. Pharmacokinetic Properties Parameter Carbimazole Propylthiouracil Absorption Peak plasma thiamazole levels within 1–2 hours Rapid GI absorption; bioavailability 50–75% Distribution Vd ≈0.5 L/kg; concentrated in thyroid; moderate protein binding; crosses placenta and enters breast milk Concentrated in thyroid; ~80% protein bound; crosses placenta and enters breast milk Metabolism Rapidly metabolised to active thiamazole Rapid first‑pass hepatic metabolism to glucuronide Elimination >90% renal excretion; ~10% enterohepatic circulation Renal excretion of glucuronide; minimal faecal excretion Half‑life 5.3–5.4 hours (shorter in hyperthyroidism; prolonged in renal/hepatic impairment) 1–2 hours (prolonged in renal/hepatic impairment) Pregnancy and Breastfeeding Treatment should only be initiated on specialist advice. Methimazole and propylthiouracil cross the placenta and enter breast mil PTU has a lower risk of severe congenital malformations than methimazole Choice of agent is based on individual risk–benefit assessmen Use the lowest effective dose to avoid fetal goitre and hypothyroidism Block‑and‑replace therapy is contraindicated in pregnancy Adverse Effects Carbimazole adverse effects usually occur within the first 8 weeks and include nausea, headache and skin rash. Propylthiouracil may cause alopecia, oedema, nausea and vomiting. Rare but serious effects (0.1–1.2%) for both agents include bone marrow suppression (neutropenia, agranulocytosis, eosinophilia, leucopenia). Patients should stop treatment and seek urgent medical advice if they develop: Sore throat Fever Mouth ulcers Bruising or bleeding Malaise Other serious adverse effects: Acute pancreatitis (carbimazole) Hepatotoxicity (both agents) Systemic vasculitis (propylthiouracil) Contraindications Carbimazole: Severe hepatic impairment Previous carbimazole‑induced pancreatitis Serious pre‑existing haematological disorders Drug Interactions Caution with drugs that cause agranulocytosis Carbimazole may increase the effect of vitamin K antagonists (e.g. warfarin) Digoxin and theophylline doses may require reduction as thyroid function normalises Iodine / Iodide Iodine is converted to iodide (I⁻), causing a temporary reduction in thyroid hormone release. High‑dose iodide improves symptoms within 1–2 days. Indications Pre‑operative preparation for thyroid surgery Post‑exposure prophylaxis following radioactive iodine exposure Iodide reduces iodination of thyroglobulin and decreases thyroid gland vascularity. Therapeutic considerations Maximal effect seen within 10–15 days Adverse effects include allergic reactions, lacrimation, conjunctivitis and salivary gland pain Key prescribing point Iodide is used as a short‑term adjunct, and long‑term use should be avoided due to loss of antithyroid efficacy. References Hall J, Guyton A. Guyton and Hall textbook of medical physiology. 13th ed. Philadelphia, PA: Elsevier; 2016. Radioiodine for thyrotoxicosis – The Christie NHS Foundation Trust [Internet]. Available from: 146-radioiodine-treatment-for-thyrotoxicosis-nm-march-2020.pdf Kumar V, Abbas A, Aster J, Robbins S. Robbins and Cotran pathologic basis of disease. 7th ed. 2005. Rang H, Dale M, Flower R, Henderson G. Rang and Dale’s pharmacology. 8th ed. 2016. Hyperthyroidism | Health topics A to Z | CKS | NICE Accessed 5/1/2026 Hyperthyroid and Pregnant – Thyroid UK Accessed 5/1/2026 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum | Thyroid® Accessed 5/1/2026 Carbimazole 5 mg Tablets – Summary of Product Characteristics (SmPC) – (emc) | 13015 Accessed 5/1/2026 Propylthiouracil Tablets BP 50mg – Summary of Product Characteristics (SmPC) – (emc) | 9177 Access 5/1/2026 Aqueous Iodine Oral Solution BP – Summary of Product Characteristics (SmPC) – (emc) | 4828 accessed 5/1/2026 Do you think you’re ready? Take the quiz below Pro Feature - Quiz Drugs For Hyperthyroidism Question 1 of 3 Submitting... Skip Next Rate question: You scored 0% Skipped: 0/3 More Questions Available Upgrade to TeachMePharmacy Pro Challenge yourself with over 2100 multiple-choice questions to reinforce learning Learn More Rate This Article