Altered patterns of thyroid hormones in serum and urine in pregnancy and during oral contraceptive therapy.

Serum total, percentage free fraction and absolute serum free hormone concentration of thyroxine and triiodothyronine were measured in control, pregnant and oral contraceptive users, together with the daily urinary losses of unconjugated thyroid hormones. Increased urinary losses of both hormones, in particular thyroxine, were apparent in pregnancy and these could not be explained in terms either of an increased filtered load of hormone or the presence of proteinuria. The possible existence of filterable small-molecular weight hormone-binding substances in the urine of pregnant patients is discussed. It is concluded that assay of urinary thyroid hormones during pregnancy is of limited diagnostic value because of overlap with thyrotoxic values.