THYROID SIZE AND THYROID-FUNCTION DURING PREGNANCY - AN ANALYSIS

An analysis of all available studies of thyroid size and function in p regnancy reveals that thyroid size, estimated by inspection and palpat ion or measured more accurately by ultrasonography, increases in pregn ancy in areas of iodine deficiency but not in those with sufficient io dine. The increase in average thyroid size is within the normal range, and can partly be prevented by treatment with extra iodine or thyroxi ne. There is a slight transient increase in free thyroxine in the firs t trimester, probably as a result of physiological stimulation of thyr oid function by human choriogonadotrophin. These levels then decrease by about 30% to low normal values in the second and third trimesters o f pregnancy in both iodine-depleted and -replete areas. These changes resemble those of non-thyroidal illness and may well play a role in re ducing energy expenditure during pregnancy. The increase in thyroid si ze in iodine-deficient areas is probably due to autoregulatory mechani sms of iodine on thyroid growth. The hypothesis is supported by the fa ct that, during pregnancy, thyroid volume and thyroid function adapt i n a physiological way to meet the increased demands for iodine and ene rgy.