1. The influence of thyroid function on the development of hypertension was studied in strains of spontaneously hypertensive (SH) and normotensive rats.

2. Surgical thyroidectomy decreased systolic blood pressure more markedly in SH rats than in normotensive rats. The effects of oral administration of 5 and 100 μg of thyroxine 24 h—1 100 g—1 were studied in the thyroidectomized animals. In the two strains the blood pressure returned to control levels only after administration of the larger dose.

3. The evolution of body weight, total plasma tri-iodothyronine (T3) and tetraiodothyronine (T4) concentrations were followed as a function of age in SH rats and normotensive rats from 5 to 21 weeks. At each age, SH rats showed significantly larger body weight and decreased T4 concentrations. Plasma T3 in SH rats was lower than in normotensive rats until 15 weeks of age, after which the difference was not significant. At 11 weeks, plasma free T3 and T4 concentrations were slightly lower in SH rats than in normotensive rats.

4. The more marked hypotensive effects of surgical thyroidectomy in SH rats cannot be related to increased thyroid function.

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