Carbimazole changes thyroid status and unpredictably alters the anticoagulant effect of warfarin, with a risk of bleeding (supratherapeutic INR) or of anticoagulant failure (subtherapeutic INR).
Hyperthyroidism increases catabolism of vitamin K-dependent clotting factors and sensitivity to warfarin; when the thionamide restores euthyroidism, warfarin requirements change (usually a dose reduction), so the anticoagulant effect may increase or decrease over the 6–8 weeks of thyroid rebalancing.
Frequent INR, TSH and free T4, haemoglobin, bleeding and thrombotic signs.
Epistaxis, gum bleeding, haematuria, melaena, sudden headache; limb pain and swelling (thrombosis).
Measure INR before starting and every 1–2 weeks while thyroid function is being adjusted, with small dose steps; the effect is bidirectional, so monitor for both bleeding and thrombosis.
Drugs.com professional monograph Methimazole/Warfarin — "Anticoagulation effects may be increased or decreased with the concomitant use of antithyroid drugs and vitamin K antagonists"; Howard-Thompson A. et al., Graves Disease and Treatment Effects on Warfarin (PMC4065757): https://pmc.ncbi.nlm.nih.gov/articles/PMC4065757/; INFARMED Prontuário Terapêutico (11th ed., 2012), 8.3 Thyroid hormones and antithyroid drugs.
Povidone-iodine applied to extensive wounds or mucosa is absorbed in sufficient amounts to affect the thyroid, potentially destabilising antithyroid therapy and interfering with thyroid function tests.
Systemically absorbed iodine load that alters thyroid response: the Prontuário (Povidone-iodine 13.1.1) states it "may be absorbed in sufficient quantity to affect the thyroid", requiring caution "in patients with thyroid disorders", and that it "may interfere with the results of thyroid function tests".
TSH, free T4, T3 and clinical signs of hypo- or hyperthyroidism.
Recently developing goitre, tachycardia, worsening hyperthyroidism, lethargy (hypothyroidism).
Avoid prolonged application to extensive or mucosal areas in patients on antithyroid drugs; prefer chlorhexidine; interpret TSH and free T4 with caution in the weeks after exposure.
INFARMED Prontuário Terapêutico (11th ed., 2012), Povidone-iodine 13.1.1 — systemic absorption affecting the thyroid and interfering with thyroid tests.
Foods very rich in iodine (seaweed, kelp, iodine supplements) can antagonise or destabilise hyperthyroidism control during antithyroid treatment.
Avoid seaweed and iodine supplements; keep your usual diet consistent.
Prontuário Terapêutico INFARMED, 8.3 (antithyroid drugs — hyperthyroid control depends on iodine balance).
Carbimazole is contraindicated in hepatic disorders — idiosyncratic hepatotoxicity can be fatal.
Assess liver function before starting; discontinue if hepatitis symptoms develop (jaundice, dark urine, pruritus).
Prontuário Terapêutico INFARMED, 8.3 — Carbimazole: "Contraindications and precautions: ... hepatic disorders, pregnancy and breastfeeding".
Agranulocytosis is carbimazole's most serious adverse reaction (rare but fatal); pre-existing neutropenia increases risk and requires haematological monitoring.
Request full blood count before starting and at any suspicion of infection (fever, sore throat); discontinue if neutropenia develops.
Prontuário Terapêutico INFARMED, 8.3 — Carbimazole: "rare but serious agranulocytosis... discontinue if evidence of neutropenia".
Thionamides (carbimazole and its metabolite thiamazole/methimazole) cross the placenta and can cause fetal and neonatal goitre, hypothyroidism and neonatal aplasia cutis (localised scalp defect). The Prontuário records "Neonatal goitre" for carbimazole (Annex 1). Uncontrolled maternal hyperthyroidism itself is associated with miscarriage, prematurity and fetal goitre — treatment should be continued at the lowest effective dose.
Use only if necessary, at the lowest effective dose, and prefer propylthiouracil in the 1st trimester (lower risk of congenital malformations with carbimazole/thiamazole in that period); switch to carbimazole in the 2nd–3rd trimesters because of PTU hepatotoxicity. Fetal surveillance (goitre, heart rate) and neonatal thyroid function.
Carbimazole/thiamazole is excreted into milk and can cause hypothyroidism and goitre in the infant ("Thiamazole... present in milk"; the manufacturer recommends avoiding). If breastfeeding is essential, use the minimum dose (≤15 mg/day) and monitor infant thyroid function; historically it is recommended to avoid.
Treat hyperthyroidism before planning pregnancy; untreated thyrotoxicosis in pregnancy is associated with miscarriage and pre-eclampsia.
Prontuário Terapêutico INFARMED — Annex 1: "Carbimazole — Neonatal goitre" (l. 39611); 8.5 (Thiamazole, p. 357); EMC-UK (MHRA) — Carbimazole SmPC: https://www.medicines.org.uk/emc/product/2175/smpc (section 4.6).
Clinical and educational support tool. The information does not replace a medical prescription or the opinion of a qualified healthcare professional.
Progressive reduction of T3/T4 synthesis (clinical onset 2–4 weeks); normalisation of thyroid function in 6–12 weeks.
Prodrug to methimazole: inhibits thyroid peroxidase (TPO), blocking thyroglobulin iodination and iodotyrosine coupling (MIT/DIT → T3/T4).
Oral: rapid complete absorption; active thyroid uptake (glandular accumulation).
Hepatic conversion to methimazole; subsequent methimazole metabolism; renal excretion.
~ 3–6 h (methimazole; longer intrathyroid persistence).
Medicines from the same therapeutic group (ATC classification) or the same pharmacological class.
