A 34-year-old G2P1 female with known RTH-beta, currently at 34 weeks gestation who has declined prenatal testing. Her mother, a maternal aunt and grandmother, are also affected. She initially presented with hair loss at age 13 years, and later developed a small goitre, intermittent palpitations and tremors. Investigations revealed elevated free T4 of 26-33 pmol/L (11-21), free T3 of 0.6-8 pmol/L (3.5-6.5) and nonsuppressed TSH of 0.66-8.1mIU/L (0.5-4.0). Genetic testing confirmed a heterozygous mutation R429Q in the THRB gene. Her first pregnancy was complicated by intrauterine growth restriction in the third trimester, with genetic testing later confirmed her daughter did not harbour the THRB mutation. During this pregnancy, her free T4 levels range from 22-26.4 pmol/L (10-19 pmol/L) and TSH 0.96-1.26 mU/L (0.5-4.0). To date, foetal monitoring has not demonstrated tachycardia, fetal goitre or growth restriction. This case raises the question of how we should best manage women with RTH-β during pregnancy?
RTH-beta is characterised by elevated thyroid hormones, nonsuppressed TSH, and variable clinical phenotype encompassing both hyperthyroid and hypothyroid features(1). High levels of thyroid hormones in pregnant mothers with RTH adversely affects normal foetuses, suppressing their TSH and producing infants with low for gestational age(2). Due to autosomal dominant inheritance, there is a 50% the foetus will have the mutation and therefore protected from the toxic effect of maternal thyroid hormone excess. Currently, there are no established guidelines for management of RTH in pregnancy. However, a suggested approach is first to determine the genotype of the foetus. For mothers carrying an affected foetus, no intervention is necessary. For mothers carrying an unaffected fetus, the aim is to maintain free T4 levels 20-50% of the upper limit of normal. Judicious use of antithyroid medication has been shown to prevent low birth weight and postnatal TSH suppression (3,4).