Measurement of thyroxin synthesis with I131; a test for evaluation of thyroid function in equivocal states.

As the function of the thyroid gland is the synthesis and secretion of thyroxin, a test which correctly measures this process is best for diagnosis of thyroid disorder and for determining the success of therapy. The rate of secretion can be measured with a Geiger counter which indicates what proportion of radioactive iodine in a serum specimen is in the form of thyroxin. The normal proportion is 2 to 10 per cent; in hyperthyroidism the proportion is 50 to 70 per cent, and in hypothyroidism less than 1 per cent. The same test has served to detect metastases of thyroid carcinoma following total thyroidectomy.