Iodothyronine Secretion from Perfused Dog Thyroid Lobes after Prolonged Thyrotropin Treatmentin Vivo*

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Abstract
In previous studies we found that a considerable part of the T3 and rT3 secreted from perfused dog thyroid lobes originates from T4 deiodinated during secretion. In the present study, we investigated the influence of prolonged TSH pretreatment in vivo on these processes. Six mongrel dogs were treated with 0.2 IU/kg bovine TSH, sc, every 12 h for 7 days before thyroid perfusion, while 6 dogs received saline. Serum T4, T3, and rT3 were considerably increased by TSH treatment, the concentrations being highest during the early part of the stimulation period. The thyroid weight of TSH-treated dogs was nearly double that of the controls, while the absolute iodothyronine content was approximately half that of the controls. The T4 concentration in effluent perfusate from the TSH-pretreated thyroid lobes was 8 times higher than that from control lobes.This was, however, significantly lower than the response to an acute infusion of 100 μU/ml TSH (19 times) seen in a large series of experiments in untreated dogs. The infusion of 100 μU/ml TSH in the TSH-pretreated thyroids did not induce any further increase in secretion. Thus, the secretory capacity of the chronically stimulated thyroids was reduced compared to that of normal dog thyroids. The T4 to T3 ratios in thyroid hydrolysate were similar in the two groups, while the T4 to rT3 ratio was significantly lower in TSH-treated thyroids. The relative concentrations of T4, T3, and rT3 in the thyroid effluent were similar in the two groups. The relative hypersecretion of triiodothyronines previously described in control thyroids was also found in the TSH-treated dogs. The hypersecretion of T3 compared to that of T4 was identical in the groups, while the hypersecretion of rT3 was reduced in the chronically stimulated thyroids. This is, however, the same pattern observed after a few hours of TSH stimulation. Accordingly, there was no evidence for any modification of intrathyroidal iodothyronine deiodination by prolonged thyroid stimulation.