Thyroid size and thyroid function during pregnancy: an analysis.

An analysis of all available studies of thyroid size and function in pregnancy reveals that thyroid size, estimated by inspection and palpation or measured more accurately by ultrasonography, increases in pregnancy in areas of iodine deficiency but not in those with sufficient iodine. The increase in average thyroid size is within the normal range, and can partly be prevented by treatment with extra iodine or thyroxine. There is a slight transient increase in free thyroxine in the first trimester, probably as a result of physiological stimulation of thyroid function by human choriogonadotrophin. These levels then decrease by about 30% to low normal values in the second and third trimesters of pregnancy in both iodine-depleted and -replete areas. These changes resemble those of non-thyroidal illness and may well play a role in reducing energy expenditure during pregnancy. The increase in thyroid size in iodine-deficient areas is probably due to autoregulatory mechanisms of iodine on thyroid growth. The hypothesis is supported by the fact that, during pregnancy, thyroid volume and thyroid function adapt in a physiological way to meet the increased demands for iodine and energy.