Endocrine & Metabolic/Thyroid & antithyroid drugs

Thyroid drugs

also: levothyroxine · carbimazole · propylthiouracil · PTU · antithyroid

Overview

Replacement for hypothyroidism (levothyroxine) and suppression for hyperthyroidism (carbimazole/PTU). The pregnancy choice and agranulocytosis warning are the exam points.

Mechanism

Levothyroxine (T4) is converted to active T3 → restores metabolic rate. Carbimazole and propylthiouracil inhibit thyroid peroxidase → block iodination/coupling → ↓thyroid hormone synthesis. PTU additionally blocks peripheral T4→T3 conversion (useful in storm).

Indications

Hypothyroidism (levothyroxine)
Hyperthyroidism / Graves (carbimazole; PTU)
Thyroid storm (PTU + others)

The agents

LevothyroxineT4 replacement

OD, empty stomach

Separate from calcium/iron/PPI by 4 h; titrate to TSH.

Carbimazoleantithyroid

First-line Graves; teratogenic (esp. 1st trimester — aplasia cutis).

Propylthiouracil (PTU)antithyroid

Preferred in 1st-trimester pregnancy + thyroid storm; hepatotoxic.

Adverse effects

Agranulocytosis (carbimazole & PTU)serious

CLASSIC — warn the patient to report sore throat/fever urgently → urgent FBC; stop drug.

Carbimazole: teratogenicityserious

Aplasia cutis / congenital malformations — PTU preferred in 1st trimester.

PTU: hepatotoxicityserious

Levothyroxine over-replacement: AF, osteoporosis, tremorcommon

Cautions & contraindications

Carbimazole in 1st-trimester pregnancypregnancy

Teratogen — switch to PTU.

Levothyroxine: caution starting in IHD/elderlyelderly

Start low — can precipitate angina/AF.

Interactions

Levothyroxine absorption ↓ by calcium, iron, PPIs, antacids
Levothyroxine raises warfarin effect

Monitoring & kinetics

TFTs: TSH (recheck ~6–8 weeks after a change); FBC if infection symptoms on antithyroid,LFTs (PTU)

Oral. Levothyroxine long t½ (weekly stability); titrate slowly.

Source: NICE NG145 — Thyroid disease · BNF — Thyroid / antithyroid