Preferential secretion of triiodothyronine in man

The directly measured secretion rates of thyroxine (T4), 3,5,3'-triiodothyronine (T3), and 3,3',5'-triiodothyronine (rT3) were compared to the tissue concentrations of these hormones and to the distribution of follicular iodoproteins in 15 patients submitted to thyroid or parathyroid surgery. The T4/T3 and rT3/T3 ratios were higher for thyroid tissue than for the secretion rates, indicating a preferential secretion of T3. The concentrations of T4 and T3 were higher in the 27S iodoprotein fraction than in the 19S thyroglobulin fraction.