NPPB and immune system disorder: However, despite being synthesized by the myocardium, serum BNP levels can be altered by non-cardiac factors, such as renal function, immune system disorders, and hypoxic conditions in addition to neuroendocrine factors that can modulate its secretion, including adrenergic agonists, glucocorticoids, endothelin-1, acetylcholine, prostaglandins, thyroid hormones, and angiotensin II.