Casey BM et al. (2006) Subclinical hyperthyroidism and pregnancy outcomes. Obstet Gynecol 107: 337–341

Among the long-term consequences of subclinical hyperthyroidism is progression to overt hyperthyroidism, which has adverse effects on pregnancy outcomes. To investigate the influence of subclinical hyperthyroidism on pregnancy outcomes, Casey et al. used a CHEMILUMINESCENT ASSAY to measure TSH levels in women who delivered a single infant weighing ≥500 g at a Texan hospital over a 2.5 year period.

Of the 25,765 women who were included in the study, 433 (1.7%) had subclinical hyperthyroidism (defined as an endogenous T4 level of ≤1.75 ng/dl and TSH levels at or below the 2.5th percentile for gestational age).

A greater proportion of African American women had subclinical hyperthyroidism than Hispanic women (3.0% versus 1.6%, P <0.001), perhaps because serum human chorionic gonadotropin levels, which are inversely related to TSH levels, are higher in black women than in Hispanic women. Patients with subclinical hyperthyroidism were more likely to have been parous previously than women with normal TSH levels (72% versus 64%, P <0.001). None of the adverse pregnancy outcomes measured were associated with subclinical hyperthyroidism. The only significant difference seen between women with subclinical hyperthyroidism and those with normal TSH was the incidence of gestational hypertension, which was significantly lower in the former group (6.0% versus 8.8%, P = 0.04).

The authors concluded that subclinical hyperthyroidism does not adversely affect pregnancy outcome among women who give birth to a single child.