Hyperthyroidism (thyroid overactivity, thyrotoxicosis) is common, affecting perhaps 2-5% of all females at some time and with a sex ratio of 5 : 1, most often between ages 20 and 40 years. Nearly all cases (> 99%) are caused by intrinsic thyroid disease; a pituitary cause is extremely rare.
Treatment of Hyperthyroidism
Three possibilities are available:
- Antithyroid drugs
- Thyroidectomy - surgery
- Radioactive iodine- Radioiodine is now more widely used in the UK, as has previously happened elsewhere, although it is contraindicated in pregnancy and while breast-feeding. Iodine-131 in an empirical dose (usually 200-500 MBq), accumulates in the thyroid and destroys the gland by local radiation - though it takes several months to be fully effective. Strict radiation safety rules apply in the UK and may be inconvenient or disconcerting for some patients. Patients must be rendered euthyroid before treatment though they have to stop antithyroid drugs at least 4 days before radioiodine, and not recommence until 3 days after radioiodine (many patients who are well controlled before radioactive iodine do not need to restart at all).
Risk of carcinogenesis has been long debated, but it is now clear that overall cancer incidence and mortality are not increased after radioactive iodine (and indeed are significantly reduced in some studies) but the risk of thyroid cancer is significantly increased although the risk remains very low in absolute terms.
Practices and beliefs differ widely within and between countries.
Hyperthyroidism in pregnancy and neonatal life
Maternal hyperthyroidism during pregnancy is uncommon and usually mild. Diagnosis can be difficult because of misleading thyroid function tests, although TSH is largely reliable. The pathogenesis is almost always Graves' disease. Thyroid-stimulating immunoglobulin (TSI) crosses the placenta to stimulate the fetal thyroid. Carbimazole also crosses the placenta, but T 4 does so poorly so a 'block-and-replace' regimen is contraindicated. The smallest dose of carbimazole necessary is used and the fetus must be monitored (see below). The paediatrician should be informed and the infant checked immediately after birth - overtreatment with carbimazole can cause fetal goitre. Breast-feeding while on usual doses of carbimazole or propylthiouracil appears to be safe.
If necessary (high doses needed, poor patient compliance or drug side-effects), surgery can be performed, preferably in the second trimester. Radioactive iodine is absolutely contraindicated.