Metabolic clearance of endogenous and radioiodinated thyroglobulin in rats

The basal concentration and metabolic clearance of radiolabeled and endogenous thyroglobulin (Tg) were measured in normal male rats. Serum Tg concentrations (by RIA) in Sprague-Dawley adult males ranged from 70-680 ng/ml (mean +/- SD, 218 +/- 177). In male Fisher rats, basal serum Tg concentrations were 32-263 ng/ml. The fractional disappearance rate of endogenously radioiodinated rat Tg and of endogenous rat Tg after thyroidectomy were both about 8%/h. The MCR of endogenously labeled Tg was 2.4 +/- 1.1 ml/100 g/h. Using this clearance and the mean serum Tg concentration in these rats, a daily thyroidal release rate of about 201 microgram Tg/100 g BW would be required to provide the T4 metabolized daily by the rat, the Tg released into the circulation comprises about 7% of the total rat Tg produced in a 24-h period.