Expansion
Levothyroxine for replacement, thionamides for suppression
Expansion
Hypothyroidism: levothyroxine
- Half-life about 7 days, so once daily and steady state after about 6 weeks
- Recheck TSH at 6 to 8 weeks after any dose change; earlier testing is uninterpretable
- Take on an empty stomach; absorption is reduced by iron, calcium, proton pump inhibitors and coffee
- Start at a low dose in the elderly and in ischaemic heart disease, since raising metabolic rate can precipitate angina or arrhythmia
- Requirements rise in pregnancy by about 25 to 50 per cent from early on
Hyperthyroidism: thionamides
- Carbimazole first line; propylthiouracil preferred in the first trimester and in thyroid storm (it also blocks peripheral conversion)
- Both take weeks to work because of the large stored hormone pool, so a beta blocker is given for symptom control meanwhile
- Agranulocytosis is the critical adverse effect: any sore throat, fever or mouth ulcer requires an immediate full blood count and stopping the drug
- Propylthiouracil carries a risk of severe hepatotoxicity
- Regimens are titration or block and replace
Definitive options are radioiodine (contraindicated in pregnancy and in active thyroid eye disease) and surgery.