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Role of the renin-angiotensin system in the development of thyroxine-induced hypertension

Objective: We evaluated the influence of chronic blockade of the renin-angiotensin system on hypertension induced by long-term thyroxine (T₄) administration. To this end, we determined the effects of chronic treatment with captopril on blood pressure, cardiac hypertrophy and other renal and metabolic variables of hypertensive hyperthyroid rats.

Methods: T₄ was administered s.c. at 0·38 µmol/kg per day and captopril was given in the drinking water (1·38 mmol/l). Both treatments were maintained for 6 weeks. Control rats received tap water. After the treatment period, the rats were placed in metabolic cages. Later, blood pressure was measured in conscious rats by intra-arterial determination.

Results: T₄-treated rats showed an increased mean arterial pressure (MAP) whereas, in rats treated with T₄ plus captopril, MAP was similar to that of the control group. Captopril did not affect the increased heart rate or ventricular weight/body weight ratio of hyperthyroid rats, but it improved the reduced creatinine clearance of these animals.

Conclusions: The elevation in blood pressure produced by long-term T₄ administration was prevented by chronic blockade of the renin-angiotensin system. Captopril improved the renal function of hyperthyroid rats, but did not affect the relative cardiac hypertrophy of these animals.

European Journal of Endocrinology 136 656–660