Influence of Perchlorate on the Secretion of Non‐Thyroxine Iodine by the Normal Human Thyroid Gland

. Several authors have postulated that endogenous iodide produced by the deiodination of iodotyrosines in the thyroid feeds into a different thyroidal iodide compartment than transported iodide which enters the gland from outside. One argument for the existence of two separate iodide compartments is the observation that under certain experimental conditions perchlorate completely discharges transported iodide from the thyroid, while it has no such effect on endogenous iodide. This latter observation however has not been confirmed by all studies and remained controversial. – We therefore reinvestigated the effect of perchlorate on the secretion of endogenous iodide by a new, sensitive method. Five normal volunteers received tracer amounts of iodide-125I p.o. and 11 days later thyroxine-131I I.V. Two days later the following serial measurements were started: serum protein-bound labelled iodine (PB125I, PB131), serum total thyroxine and urinary excretion of 125I, 127I and 131I. – In the control period the non-thyroxine iodine secretion calculated from the above measurements was 40 μg/day. Under perchlorate 200 mg three times daily this value rose significantly to 66 μg/day. Non-thyroxine iodine comprises the secreted triiodothyronine plus the secreted endogenous iodide. Assuming that the former value remained constant, our data show that perchlorate indeed discharges part, though not all, of the endogenous iodide. These data do not rule out a second iodide compartment, but they are also compatible with a simple one compartment model.