Aldosterone, the main mineralocorticoid hormone, is a steroid hormone produced by the zona glomerulosa of the adrenal cortex in the adrenal gland. It is a part of the renin-angiotensin-aldosterone system (RAAS), a hormone system that regulates blood pressure and fluid and electrolyte balance, as well as systemic vascular resistance. Competitive ELISA is an immunoassay format used for the determination of low-molecular-weight targets. Typically steroid and/or thyroid hormones, vitamins and food contaminants are measured by competitive ELISA. Since the method is fast and easy to perform, it is is also frequently applied in the determination of peptides, proteins and other high-molecular-weight analytes.