TRACING THE THYROID HORMONE IN PERIPHERAL TISSUES*†

Abstract

The thyroid gland transforms inorganic iodine to organic combinations thereof. Both the blood plasma and peripheral tissues contain organically bound iodine which rises and falls with fluctuations in thyroid activity and with the resulting changes in basal metabolic rate. In muscle, as in the plasma, this organic I is unevenly distributed am ong the several protein fractions. In plasma and in skeletal muscle, too, the native organic I behaves unlike free thyroxine during the acetone-precipitation of proteins. After the admn. of radioiodide in tracer dosage, the activity resides at first chiefly in the inorganic fraction but, later, part of the activity becomes attached to protein. For this colloidally bound I in tissues the name "thyrenzyme" is suggested. In myxedema it disappears, but in hyperthyroidism it increases to several-fold the normal concn.