Plasma concentrations of atrial natriuretic peptide and cyclic guanosine monophosphate in patients with hyperthyroidism before and after short-term treatment with methimazole.

The aim of the study was to answer the question whether a rapid decrease in serum triiodothyronine (T3) and thyroxine (T4) levels resulting from the treatment with a full dose (3 x 20 mg daily) of methimazole applied in patients with thyrotoxicosis is associated with the parallel diminution of plasma atrial natriuretic peptide (ANP) and its second messenger-cyclic guanosine monophosphate (cGMP) concentrations. Sixteen patients with thyrotoxicosis of mean age 41.5 +/- 10.5 years participated in the study. Short-term (10 days) methimazole treatment resulted in a significant decrease in serum T3 and T4 concentrations to the values found in 14 healthy subjects serving as control group. Plasma ANP and cGMP levels also decreased significantly during the treatment attaining the normal range. A significant correlation was found between the decrease in serum T3 and T4 concentrations during the treatment and the decrease in plasma ANP level. The decrease in plasma ANP was not closely correlated with the reduction of cGMP levels. These results indicate that: 1) a steep decrease in serum thyroid hormone concentrations induced by a full methimazole treatment during ten days in patients with thyrotoxicosis due to Graves' disease was accompanied by the return of elevated plasma ANP levels to normal range; 2. diminution of serum concentrations of both T3 and T4 during the treatment was correlated with the decrease in plasma ANP; 3) reduction in plasma cGMP concentration associated with short-term methimazole treatment in thyrotoxicosis seems to depend not only on the diminution of plasma ANP level.