Effects of Altered Thyroid Function on Plasma Prolactin Clearance*

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Abstract
This study was designed to examine directly whether changes in thyroid function alter the plasma clearance and/or distribution of PRL in rats. Tracer doses of rat [125I]PRL were injected into the jugular veins of hypothyroid, hyperthyroid, and euthyroid female rats, and blood samples were drawn from the opposite carotid artery at frequent intervals over a 40-min period. The disappearance of the immunoprecipitable radioactive material from the plasma was plotted against time, and a computer program based on the Gauss-Siedel iteration algorithm was then used to derive specific equations for each of the multiexponential curves produced. It was observed that biexponential functions of the form Ct = A-αt + B-βt fit the data points of the clearance curves of each of the animals with a coefficient of determination (r2) greater than 0.94. It also became apparent with the establishment of a biexponential form for the clearance curves that PRL clearance could be suitably defined by a two-compartment open model. Therefore, the parameters and variables of this model were determined for each animal by incorporating the coefficients and slopes of our clearance curves into the equations of the model. A noncompartmental method of analysis was additionally used, where possible, to confirm independently the results obtained. Both methods of analysis clearly demonstrated that the MCR and total distribution volume of PRL in the hypothyroid rats were significantly less than those in controls. It was concluded that these changes in MCR and total distribution volume may significantly contribute to the occurrence of hyperprolactinemia in hypothyroidism.