Evaluation of serum triiodothyronine and adjusted triiodothyronine (free triiodothyronine index) in pregnancy.

We measured serum thyroxine (free and total), triiodothyronine (free and total), thyroxine-binding globulin, and triiodothyronine uptake by talc in 97 normal men and 50 pregnant women. Mean serum thyroxine and triiodothyronine concentrations were higher in the pregnant subjects (104 vs. 78 mug/liter and 1.69 vs. 1.30 mug/liter) because of a higher mean thyroxine-binding globulin concentration (70 vs. 38 mg/liter). Mean triiodothyronine uptake by talc was lower in the pregnant subjects (0.82 vs. 1.03). Mean free thyroxine concentrations were similar in the two groups, but mean free triiodothyronine concentrations were 10% lower in the pregnant subjects. Triiodothyronine uptake by talc and the diayzable thyroxine and triiodothyronine fractions were highly correlated (r = 0.85 and r = 0.82, P less than 0.001). Calculated free thyroxine index and free triiodothyronine index values (hyroxine and triiodothyronine indirectly adjusted, using triiodothyronine talc uptake to compensate for differences in thyroxine-binding globulin concentration), were statistically similar (84 vs. 82 and 1.38 vs. 1.34) in pregnant and male subjects. The results indicate that the total triiodothyronine concentration can be normalized on the basis of the triiodothyronine uptake by talc to correct for variations in thyroxine-binding globulin concentration.