While the commonest cause of hyperthyroidism in pregnancy (which affects up to 0.2% of pregnant women) is Graves' disease (85-90%), other
Table 3. Causes of hyperthyroidism in pregnancy
Transient gestational hyperthyroidism (associated with hyperemesis gravidarum)
Toxic multinodular goitre
TSH receptor activation causes such as hyperemesis gravidarum, toxic multinodular goitre, toxic adenoma and subacute thyroiditis may occur. It should be noted that most women with nausea and vomiting in pregnancy do not have hyperthyroidism. Rarer causes include struma ovarii, hydatidiform mole and one reported case of a TSH receptor mutation activated only during pregnancy  (table 3).
The clinical suspicion of hyperthyroidism may not be obvious as symptoms of tachycardia, sweating, dyspnoea and nervousness are seen in normal pregnancy as are cardiac systolic flow murmurs. The diagnosis should always be confirmed by estimation of circulating thyroid hormone concentrations. It should be noted that serum thyroxine (both total and free) varies during normal gestation. Recent national and internationally agreed guidelines suggest that laboratories should be encouraged to develop normal ranges for total but more particularly free T4 and T3, as well as TSH after the 1st trimester during pregnancy, all of which may change during the course of gestation. Normally the TSH is suppressed in hyperthyroidism but in early pregnancy (approx. 9-12 weeks) TSH is usually suppressed by human chorionic gonadotrophin and may also be lowered due to non-specific illness such as vomiting as well as multiple pregnancy. This may lead to uncertainty in differentiating Graves' hyperthyroidism from gestational thyrotoxicosis due to hyperemesis gravidarum. The diagnosis of Graves' disease may be confirmed however by demonstrating the presence of TSH receptor stimulating antibodies which are also useful markers in the management of the condition.
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