Hormone

A hormone is a chemical messenger produced by an endocrine gland and released into the bloodstream to regulate the activity of specific target cells or organs. Hormones can affect a wide range of bodily processes - for example, insulin (from the pancreas) signals cells to take up glucose, and thyroid hormone regulates metabolism.

• Hormones only act on cells that have receptors for them. Even though hormones circulate throughout the body via blood, only target cells with the appropriate receptor will respond (e.g., estrogen affects estrogen-receptor-positive tissues).
• The major classes of hormones include peptide/protein hormones (water-soluble, e.g. insulin), steroid hormones (lipid-soluble, e.g. cortisol, sex hormones), and amine hormones (modified amino acids, e.g. adrenaline). This influences how they act: e.g., steroid hormones cross cell membranes and bind intracellular receptors, whereas peptide hormones bind to cell surface receptors.
• Hormone levels are usually controlled by feedback loops. For instance, low blood thyroid hormone triggers the pituitary to release TSH, stimulating the thyroid gland to produce more hormone (negative feedback once levels normalize).

• Commonly appears in questions about distinguishing hormones from other signaling molecules. For example, a question might contrast a neurotransmitter (released at a synapse) with a hormone (released into blood) - knowing that hormones travel via bloodstream to distant targets is key.
• May be tested in scenarios involving endocrine disorders: e.g., recognizing that excessive thirst and urination in diabetes are related to lack of insulin hormone action, or that growth abnormalities could be due to growth hormone issues.