The relationship of thyroxine turnover to other parameters of peripheral thyroid function in Graves' disease

In 28 patients with Graves' disease showing a wide range of thyroid function between the extremes of hypothyroidism and hyperthyroidism, the following parameters of peripheral thyroid function were measured: serum thyroxine concentration, serum-free thyroxine concentration, serum triiodothyronine concentration, and serum-free triiodothyronine concentration. In 25 patients, thyroxine turnover was also measured. Thyroxine turnover was found to be highly correlated with serum-free thyroxine concentration (r equals 0.9405) and serum-free triiodothyronine concentration (r equals 0.9184). Serum-free thyroxine fraction correlated with serum-free triiodothyronine fraction (r equals 0.8445), suggesting that similar factors in serum controlled the intensity of protein binding for both thyroxine and triiodothyronine. Thyroxine turnover calculated by a noncompartmental method agreed closely with values calculated by the compartmental method, suggesting that the former simpler method has general utility.