To evaluate a theoretical model for calculating free thyroid hormones, based on the law of mass action, we compared values for both calculated and measured free thyroxin and free triiodothyronine in a group of normal subjects. To determine whether the concentrations of circulating free hormones were also predictable from equilibrium considerations in abnormal states, we compared calculated and measured free thyroid hormone values in an additional population including pregnant women and hyperthyroid, hypothyroid, and "sick euthyroid" patients. Predictions based on the model were accurate in all these states except pregnancy, where there was some discrepancy between calculated and measured values for both hormones. In pregnant women with large abnormalities in thyroid-hormone-binding proteins, euthyroidism was accompanied by significantly lower free hormone concentrations, leading us to conclude that, in pregnancy, equilibrium may be reached at concentrations lower than those in other healthy subjects. Values for calculated and measured free thyroid hormones in "sick euthyroids" showed no discrepancy; however, we cannot exclude the possibility that non-dialyzable compounds are present that interfere with the hormone-protein binding.