The concentration and significance of the butanol-extractable I131 of serum in patients with diverse states of thyroidal function.

Thyroxine, if not truly the active thyroid hormone ( 1, 2) is clearly the principal iodine-containing compound released by the thyroid gland, and ordinarily comprises the largest fraction of the plasma's protein-bound iodine (3-6). Many investigators have, therefore, attempted to assess thyroidal function by measuring the concentration of protein-bound I'll (PB1181) in the plasma following the administration of radioactive iodine (7-11). The PBI111 (12), or even the total radioactivity in the plasma (13, 14), determined several days after the administration of the tracer, reflects the radioactivity of circulating thyroxine, except in patients who have received large doses of 1181 (15). It has been shown that the hormonal iodine which is extractable from serum with butanol and not re-extractable with alkali (hereafter referred to as the butanol-extractable iodine) is composed largely of thyroxine. Its concentration in serum differs significantly from that of the protein-bound iodine (16). The present communication reports the results in 109 patients with varying states of thyroidal function in whom the plasma's concentration of radioactive butanol-extractable iodine was determined both one and three days after the administration of 181. From a correlation of these results with measurements of the amount of 11l8 accumulated by the thyroid gland, an estimate of both the amount and the rate of turnover of glandular hormone has been made.