The uptake of 131I by the thyroid gland of the castrated adult male rat is increased 24 hr. and is maximal 48 hr. after the injection of a single dose of 50 μg. oestradiol. The response is not dose-dependent between 25 and 1600 μg.
The thyroid:serum (T:S) concentration ratio for 131I is also increased by oestradiol with a time-course similar to the changes in uptake, but release of 131I-labelled hormone from the gland in vivo and radioactive phosphate uptake were not affected.
Analysis of the kinetics of 131I accumulation by the blocked gland show that the effects on 131I uptake and T:S ratio were due to an increase in the clearance rate with a possible associated decrease in the exit rate constant for iodide from the gland to the blood.
Under the conditions of these experiments, the effects of oestradiol are not consistent with their being produced by an increase in pituitary thyrotrophic hormone secretion; a direct action on the gland appears likely.
These conclusions apply solely to the experimental conditions described here and do not provide the basis for any generalization about the action of oestrogens on the thyroid gland.
The method of analysis developed for the estimation of the unilateral clearance constant and the exit rate constant, together with their standard deviations, is presented in an appendix.
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