Thyroid disorders in pregnancy

Definition

• Thyroid disorders in pregnancy comprise hypothyroidism, hyperthyroidism and thyroid autoimmunity; after diabetes, they are the most common endocrine disorders of pregnancy.

Classification

• Transient gestational thyrotoxicosis results from direct hCG stimulation of the thyroid, usually in the first trimester, including in hyperemesis gravidarum or multiple pregnancy.

Aetiopathogenesis

Aetiology and pathogenesis

• hCG has weak TSH receptor-stimulating activity; with the hCG peak at 10–12 weeks of gestation, TSH may be reduced in the first trimester.
• By 6–8 weeks of gestation, thyroxine-binding globulin rises by about 50%, with a corresponding rise in total T4 and T3; iodine requirements increase.
• The commonest cause of hypothyroidism is chronic autoimmune (Hashimoto's) thyroiditis, and the commonest cause of overt hyperthyroidism, at 85%, is Graves' disease.

Clinical features

Clinical features and complications

• Overt hypothyroidism is associated with preterm birth, low birthweight, perinatal death, pre-eclampsia, placental abruption and anaemia.
• TSH receptor antibodies can cross the placenta and stimulate the fetal thyroid at high concentrations; fetal hyperthyroidism is rare, at 1 in 4,000–40,000 pregnancies.

Diagnosis

• Overt hypothyroidism is defined as TSH above the trimester-specific range with low fT4.
• TSH receptor antibodies have high sensitivity and specificity for Graves' disease.
• Ultrasound features suggestive of fetal thyroid dysfunction are goitre, growth restriction, hydrops, tachycardia or heart failure.

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Further reading (selection)

1. J Clin Transl Endocrinol 2016: Benign thyroid disease in pregnancy: A state of the art review (PubMed Central)

Cross-references