Human chorionic gonadotrophin (hCG) has the same alpha subunit as thyroid stimulating hormone (TSH). Maternal TSH production is suppressed during the first trimester, when hCG levels are the highest. The TSH response to TRH (thyrotrophin releasing hormone) is reduced during the first trimester then returns to normal after this. Total T3 and T4 production increase because of the increased production of thyroid binding globulin (TBG) which starts in the first 2 weeks and reaches plateau by 20 weeks. Thyroid hormone reference ranges for nonpregnant women are not appropriate in pregnancy. Trimester specific range of free T4, free T3 and TSH should be analysed. There is a fall in TSH and a rise free T4 concentrations in 1st trimester followed by fall in fT4 concentration with advancing gestation. Iodine is essential for thyroid hormone synthesis. There is increased renal loss of iodide in pregnancy. The thyroid compensates by increasing uptake of iodide from circulation. Thyroid gland becomes enlarged where there is background iodine deficiency.
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