This study was designed to examine the involvement of thyroid hormone in the 
release of brain natriuretic peptide (BNP) from the heart. We measured plasma 
immunoreactive BNP (ir-BNP) concentrations in patients with untreated 
hyperthyroidism. We also measured BNP values in experimental rats with 
hyperthyroidism induced by thyroxine (T4) and in rats with hypothyroidism 
induced by propylthiouracil (PTU). The in vitro effects of triiodothyronine (T3) 
and T4 on the release of BNP were examined in newborn rat atrial and ventricular 
myocytes in primary culture. Plasma BNP levels were increased in hyperthyroid 
patients compared with normal control subjects. Plasma BNP levels were increased 
in hyperthyroid rats and decreased in hypothyroid rats compared with euthyroid 
rats. Plasma BNP level was correlated with serum T4 level in hyperthyroid 
patients and hyperthyroid rats. A major component of ir-BNP in plasma from 
hyperthyroid patients was human BNP-32 and that in plasma from hyperthyroid rats 
was rat BNP-45. T4 and T3 stimulated release of ir-BNP from both cultured atrial 
and ventricular myocytes in a dose-dependent manner. Plasma BNP concentration is 
frequently increased in hyperthyroidism, and thyroid hormone may regulate BNP 
release from both atrial and ventricular myocytes.