Circulating thyroid hormone is measured more accurately by determination of serum precipitable iodine than of basal metabolic rate (1 to 8). Since the latter increases during pregnancy (9 to 14) serum precipitable iodines were investigated in 43 pregnant women and followed in some subjects after delivery.'A preliminary report of this investigation was included in the

In 1948 it was reported by Heinemann, Johnson and Man (1) that early in pregnancy the precipitable (protein-bound) iodine in the serum (SPI) rose to concentrations in the upper range of normal or even to levels that outside of pregnancy are characteristic of hyperthyroidism. It also appeared that if the SPI did not rise, the pregnancy was likely to end in abortion within the first four months. Some evidence was presented that such accidents might be prevented by the administration of active thyroid substance. The present paper deals with a further investigation of the phenomenon.

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The relation of thyroid activity to sex and its functions has long been recognized, but the particulars of this relationship have been subjects of endless speculation and conflicting hypotheses. Before presenting the few additional facts we have to offer, it may be well to review the facts already available and current opinions on the subject. Women are undoubtedly more susceptible than are men to all recognized functional disturbances of the thyroid gland, especially hyperactivity of the organ. The incidence of this latter condition appears to be greater during periods of changing sexual function-adolescence and the menopause. Concerning the effect of pregnancy there is some difference of opinion. It is generally agreed that the thyroid gland undergoes hyperplasia during pregnancy, but there is no certainty that this hyperplasia is attended by hyperfunction. The basal metabolism rises in the latter months of pregnancy,8, 10, 19, 21 but the rate of oxidation is not controlled by the thyroid gland alone. The rise of basal metabolism in pregnancy is not accompanied by other recognized manifestations of hyperthyroidism. Indeed, serum cholesterol is said to increase, a phenomenon that is regularly observed in hypothyroidism.1'4'8 The hypermetabolism of pregnancy is not regularly accompanied by the circulatory or nervous manifestations that characterize hyperthyroidism.Clinical hyperthyroidism may begin during pregnancy, but this does not seem to be a frequent occurrence.10 Clinical hyperthyroidism does not appear to influence the normal course of pregnancy nor to prevent conception. Thyroidectomized female animals are unable to conceive. Complete absence of the gland produces sterility in both sexes. This has given rise to the impression that infertility in women with normal menses may be due to some lesser degree of thyroid deficiency. Convincing evidence for such a concept has been hard to find, chiefly be-

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