Annotations

A hormone is classically defined as a physiological regulator synthesised by ductless glands and trans-ported in plasma to act on target cells at a distant site. How the specificity of action is conferred remained unknown until target tissue was shown to contain receptors to which the hormone bound. It is now accepted that all classes of steroid hormones, including the vitamin D sterols, act by binding to intracellular receptor proteins.1 Mechanism of action The non-protein bound or 'free ' fraction of steroid in plasma enters target cells by passive diffusion where a specific, high affinity, low capacity cytoplas-mic protein binds the steroid. The binding affinity, a measure of how avidly a receptor binds minut