Background: Hyperthyroidism is associated with several kinds of changes in the circulatory system, including alterations in blood volume. Arginine-vasopressin (AVP) is known to be one of the major factors regulating plasma volume. The present study was designed to evaluate the plasma AVP level in patients with hyperthyroidism during treatment.
Material/Methods: AVP was measured under basal conditions and after stimulation with a low-sodium diet and upright position. Seventy-four patients with hyperthyroidism and 37 controls were investigated. Measurements were taken before treatment, two weeks after pharmacological treatment, and after attaining euthyroid status. The following indices were determined: AVP, total and free thyroxin and triiodothyronine, thyrotropin, sodium, potassium, hematocrit, arterial blood pressure, cardiac index, and total peripheral resistance index. Plasma osmolality and changes in plasma volume were calculated indirectly.
Results: Plasma AVP was higher in patients with hyperthyroidism before treatment. After normalization of thyroid status, the AVP level was similar to that of the controls. The application of a low-sodium diet and upright position resulted in a greater decrease in plasma volume than the controls. AVP correlated with thyroxin level and plasma osmolality in patients with hyperthyroidism.
Conclusions: Enhanced AVP level in patients with hyperthyroidism is suggested to be the result of alterations in plasma volume and is relatively independent of changes in plasma osmolality.

Keywords: Arginine-vasopressin, hyperthyroidism, plasma osmolality, treatment