Objective and patients: To further explore the difference in plasma noradrenaline in normotensive and hypertensive hypothyroid patients we have investigated the pressor response to exogenous noradrenaline in 11 normotensive and five hypertensive patients with primary hypothyroidism before and after thyroxine replacement. Seven healthy subjects served as controls.
Design: The patients were studied under metabolic ward conditions and received a Na+ and K+ defined diet for 4 days. The controls received the same diet on an ambulatory basis for 3 days and were admitted to the ward in the evening on the third day. In the morning of day 4 a graded noradrenaline infusion was given. When the increase in systolic blood pressure in two consecutive registrations was at least 20 mmHg as compared to basal values the noradrenaline infusion was stopped. The dose required to increase systolic blood pressure by 20 mmHg (I20) was determined.
Results: During hypothyroidism the I20 was 120 ng/kg BW/min in normotensive patients and 39 in hypertensive patients as compared to 62 in controls. The I20 was higher in normotensives as compared to hypertensives (P = 0.041). The I20 was not different in hypertensives as compared to controls. When the patients had become euthyroid I20 decreased to 51 ng/kg BW/min (P = 0.04) in the normotensives, but remained unchanged in the hypertensives. There was no difference in I20 between normotensive and hypertensive patients in the euthyroid state, or when compared to controls.
Conclusion: The pressor response to noradrenaline was decreased in normotensive hypothyroid as compared to hypertensive hypothyroid patients, indicating a decreased peripheral sensitivity to noradrenaline in normotensive hypothyroid patients. Following thyroxine replacement the decreased response became normal.