Figure 2. Association of Thyroid Function Test Abnormalities With Preterm Birth.

All analyses were adjusted for maternal age, body mass index, ethnicity, smoking, parity, gestational age at blood sampling, and fetal sex. Euthyroid was defined as the 2.5th-97.5th cohort-specific percentile for thyrotropin (often referred to as thyroid-stimulating hormone or TSH) and free thyroxine (FT₄) concentrations; subclinical hypothyroidism, increased thyrotropin concentration with a normal FT₄ concentration; subclinical hyperthyroidism, decreased thyrotropin concentration with a normal FT₄ concentration; overt hyperthyroidism, decreased thyrotropin concentration with an increased FT₄ concentration; and isolated hypothyroxinemia, a normal thyrotropin concentration with a decreased FT₄ concentration. These clinical entities were calculated for cohorts that had thyrotropin concentration, FT₄ concentration, and thyroid peroxidase antibody data available. Absolute differences and corresponding 95% CIs were back-calculated from the results of multivariable models and adjusted for baseline risk imprecision.

^aThere were too few samples to conduct a reliable analysis.