Steroids are lipophilic, low-molecular weight compounds derived from cholesterol that play a number of important physiological roles. The steroid hormones are synthesized mainly by endocrine glands such as the gonads (testis and ovary), the adrenals and (during gestation) by the fetoplacental unit, and are then released into the blood circulation. They act both on peripheral target tissues and the central nervous system (CNS). An important function of the steroid hormones is to coordinate physiological and behavioural responses for specific biological purposes, e.g. reproduction.. Because of their lipophilic properties, free steroid molecules are only sparingly soluble in water. In biological fluids, they are usually found either in a conjugated form, i.e. linked to a hydrophilic moiety (e.g. as sulfate or glucuronide derivatives) or bound to proteins (non-covalent, reversible binding).