Review Question - QID 103474

During pregnancy, increased circulating levels of thyroxine-binding globulin (TBG) and plasma volume will increase the demand for T4. The dose of levothyroxine should be increased by 25-50% in pregnant women.

While euthyroid women can increase TSH production to meet elevated demands during pregnancy, hypothyroid patients are unable to increase TSH and need additional hormone replacement therapy. TBG levels increase due to elevated levels of circulating estrogen, subsequently decreasing the availability of free T4.

Bishnoi et al. review thyroid disease in pregnancy. Both hyperthyroidism and hypothyroidism pose risks to the fetus. Hypothyroidism is associated with maternal hypertension and premature labor, as well as low birth weight. Thyroxine replacement therapy is used to maintain serum TSH levels in the normal range throughout pregnancy.

Stagnaro-Greene et al. write that the thyroid gland must increase levels of circulating T3 and T4 roughly 50% during pregnancy due to concomitant increases in circulating TBG. Nutritional iodine requirements are elevated as well. Subclinical hypothyroidism must be evaluated also as this condition is associated with miscarriage.

Illustration A diagrams the roles of the hypothalamus, pituitary gland, and thyroid gland in normal thyroid physiology.

Incorrect Answers:
Answer 1: Cytochrome P-450 activation is not closely related to circulating levels of T3 and T4.
Answers 3 and 5: In pregnant women with hypothyroidism, TSH remains stable but cannot increase to meet the higher TSH demands of pregnancy. TSH level is therefore not decreased but remains the same, while the anterior pituitary is unable to appropriately respond to circulating T4 levels due to the inadequate TSH response.
Answer 4: Oral bioavailability of levothyroxine is not decreased in pregnant women.