Introduction

The thyroid gland undergoes changes during and following pregnancy which have been repeatedly described. These widely reported changes are essential for the understanding of the various thyroid affectations that are special for the pregnant patient. The commonest cancer of the thyroid gland, i.e. the well-differentiated variety, is frequent in females of fertile age. It is still undecided whether thyroid cancer diagnosed during pregnancy represents as a coincidental or a cause-related phenomenon. The effect of pregnancy on the behaviour of such a cancer is of some importance.

Changes in thyroid status with pregnancy

In pregnancy there is a 40% increase in maternal blood volume which contributes to an increased glomerular filtration rate and increased renal iodide clearance which produces in essence a decline in maternal serum iodide which reaches its maximum effect in the second trimester. Such a deficit is coupled with increased thyroid iodine clearance. Radioiodine thyroid uptake is increased during pregnancy and is consistent with some histological studies which have shown thyroid follicular cell hypertrophy and hyperplasia. Goiter formation can develop as a compensatory mechanism in iodine deficient areas. Fetal iodine stores reflect the state of maternal iodine deficiency.

Thyroxine binding globulin (TBG) increases more than twofold during pregnancy, with its maximum level being detected at the end of the first trimester. This increase is due to estrogen secretion by the placenta and increased serum TBG due to production of variants which are cleared from the circulation more slowly than normal forms of TBG.