Thyroid volume was measured by ultrasonography in 50 healthy adults (25 males, 25 females, age 20-70 years) living in the non-iodine deficient area of Amsterdam. Thyroid volume was 10.7 +/- 4.6 ml (mean +/- SD, range 2.7-20.4 ml). No relation was found between thyroid volume and any of the following parameters: plasma TSH, T4, T3, thyroglobulin (Tg), urinary iodine excretion, tobacco and alcohol consumption. Thyroid volume in males (12.7 +/- 4.4 ml) was greater than in females (8.7 +/- 3.9 ml, p = 0.0014), but no sex difference was observed in the ratio of thyroid volume to body weight (males, 0.16 +/- 0.05 ml/kg; females 0.13 +/- 0.06 ml/kg; NS). Thyroid volume was positively related to body weight, but not to age. We conclude that the sex difference in thyroid volume is due to the difference in body weight between males and females. Lean body mass is presumably the most important physiological determinant of thyroid size in subjects living in a non-iodine deficient area.
After intravenous injections, we have found the immunological half-life of purified porcine proinsulin to be more prolonged than purified porcine single-component insulin in both swine (22 and 9 min, respectively) and in baboons (18 and 8 min, respectively). Studies in humans have also indicated a longer half-life of proinsulin than insulin (1). These findings have prompted the present investigation of the in vitro degradation of insulin, proinsulin, and the connecting peptide that links the A and B chains of insulin in the proinsulin molecule, using an isolated liver perfusion system.Methods. Intact livers, weighing 4.5-6.8 g, from male Wistar rats, fasted for 48 hr, were cyclically perfused by the method of Hems et al. ( 2 ) . The perfusion fluid consisted of washed human erythrocytes, 2 g/lOO ml bovine serum albumin (Cohn, Fraction V ) , and Krebs-Henseleit buffer, pH 7.4, with a hemoglobin concentration of approximately 3 g/lOO ml. After complete isolation from the circulation, the livers were perfused in situ by adjusting the hydrostatic pressure of the perfusate to give maximal perfusion rates without liver swelling. This was approximately 2-3 ml/g wet weight of liver. The total perfusion volume was 150 ml at the beginning of the experiment. After a 40-min equilibration period, purified porcine single-component insulin (Lilly ), proinsulin (Lilly ), or con-
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