During the first trimester of pregnancy, serum estradiol concentration increases and the amount of thyroxine-binding globulin approximately doubles. Thus, an increase in total T4 is required to maintain free T4 within normal range. Human chorionic gonadotropin from the placenta typically stimulates the thyroid to increase production of T4. However, women with primary hypothyroidism cannot keep up with the increased demands on the thyroid during pregnancy, often resulting in maternal hypothyroxinemia in the first trimester, which may lead to impaired intellect and cognition in offspring.1 Hypothyroidism can be safely treated during pregnancy with levothyroxine. Levothyroxine dose should be increased by about 30% to 50% early in the first trimester in order to maintain serum TSH concentration between 0.5 and 2.0 µIU/mL and serum free T4 concentration within the upper third of normal.2 There is no role for treatment with T3 during pregnancy.
- Levothyroxine should be increased.
1. Haddow JE, Palomaki GE, Allan WC, et al. Maternal thyroid deficiency during pregnancy and subsequent neuropsychological development of the child. N Engl J Med 1999;341:549-55.
2. Alexander EK, Marqusee E, Lawrence J, et al. Timing and magnitude of increases in levothyroxine requirements during pregnancy in women with hypothyroidism. N Engl J Med 2004;351:241-9.
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