Thyroid physiology (Figure 47.1)
- Circulating thyroxine (levothyroxine, T4) and l-triiodothyronine (T3) are bound primarily to thyroxine-binding globulin (TBG) with <1% circulating as free (biologically active) hormone.
- Iodine is required for thyroid hormone production and fetal thyroid function is dependent on iodine from the mother.
- Non-thyroid medical illnesses and select drugs can affect thyroid function.
Thyroid function during pregnancy
- Estrogen has two effects on thyroid function in pregnancy:
1 It increases circulating TBG concentrations resulting in elevated levels of total T4 and T3
2 It increases TBG sialylation which reduces hepatic clearance of T4 and T3.
Despite these changes, circulating levels of free T4 and T3 remain unchanged.
Despite these changes, circulating levels of free T4 and T3 remain unchanged.
- Less than 0.1% of thyroid hormone crosses the placenta. As such, tests of fetal thyroid function (although rarely, if ever, indicated) are reliable and independent of maternal thyroid status.
- Thyroid hormone can be measured in fetal blood as early as 12 weeks’ gestation.
Maternal hyperthyroidism (thyrotoxicosis)
Incidence
This is 0.05–0.2% of pregnancies.
