Atrial natriuretic peptide and cyclic guanosine monophosphate plasma concentrations in patients with thyrotoxicosis and atrial fibrillation. Effect of short-term methimazole therapy

Acute Experimental Hyperthyroidism Does Not Affect Basal and Volume-Induced Atrial Natriuretic Peptide Secretion in Healthy Subjects

Background. Excess circulating thyroid hormones are associated with increased cardiac atrial natriuretic peptide (ANP) secretion but the exact mechanisms involved have not been fully elucidated in vivo. Methods. To examine whether thyroid hormone regulation of ANP secretion is the result of a direct action on the myocardium and/or of an indirect action through alterations in the peripheral circulation, plasma ANP levels (baseline and volume expansion-induced) were evaluated in 14 healthy men, before and after triiodothyronine (T₃) administration. Results. T₃ administration was followed by a significant increase in serum T₃ levels and a significant decrease in serum TSH levels, without significantly affecting ANP levels. Systemic vascular resistance, plasma rennin activity (PRA), and aldosterone (ALDO) levels, as well as indices of left atrial function, were not significantly altered, despite a significant increase in cardiac output. Plasma volume expansion, induced by a 1500 ml normal saline (NSal) infusion, both before and after T₃ administration, was followed by a significant decrease in PRA and ALDO and a significant increase in plasma ANP levels, without significantly affecting the mean blood pressure (BP) and heart rate (HR) in each study period. The NSal-induced response, measured as the integrated area under the curve corrected for baseline values (-AUC), was not different after T₃ administration for ANP, ALDO, PRA, HR, and mean BP. Conclusion. In vivo thyroid hormone-induced myocardial ANP secretion is the result of an indirect action mainly through hemodynamic changes that increase atrial stretch.

Serum concentrations of BNP and ANP in patients with thyrotoxicosis

Serum BNP (brain naturiuretic peptide) and ANP (atrial natriuretic peptide) levels are reportedly elevated in patients with thyrotoxicosis. The increases may not be due to thyrotoxicosis itself but to secondary cardiovascular changes such as chronic heart failure (HF) or atrial fibrillation (AF) which frequently accompany thyrotoxicosis. We measured serum ANP and BNP levels in 130 patients with thyrotoxicosis and correlated them with HF severity and thyroid function. Thirty-seven normal subjects served as controls. Serum BNP levels in thyrotoxic patients were significantly higher than those in control subjects and significantly correlated with serum free T4, free T3 and ANP levels. In untreated Graves' disease serum BNP level was significantly elevated in patients with HF or AF. Multiple regression analysis revealed that HF, free T4, female gender and AF are independent contributing factors to the elevated BNP level, and that these four factors contributed about 40%. On the other hand, HF and AF were contributing variables for ANP level but the overall contribution of these factors was only 10%. After normalization of thyroid function, serum BNP levels were normalized in 70.5% of Graves' patients. BNP level in euthyroid state was dependent on the presence of HF and the BNP value before therapy, but not on thyroid hormone levels or AF. These data suggest that the cardiovascular condition is the major factor responsible for the elevated serum BNP and ANP levels in thyrotoxic patients, while thyrotoxicosis itself is an independent but minor contributing factor. Thus, the determination of serum BNP levels in thyrotoxic patients is useful for monitoring cardiovascular conditions of HF.