endocrine

Meaning: Endocrine signaling uses hormones released by ductless glands (for example the pituitary, thyroid, adrenals, pancreatic islets and gonads) directly into the blood. The blood carries the hormone throughout the body, but only cells with the matching receptor respond. Peptide hormones such as insulin are water-soluble, bind cell-surface receptors and typically act through second messengers like cAMP, so effects are fast and short-lived. Steroid hormones such as cortisol and estrogen are lipid-soluble, cross the membrane, bind intracellular receptors and change gene transcription, so effects are slower and longer-lasting.

On the MCAT: Passages ask you to classify a signal as endocrine, paracrine, autocrine or exocrine, to follow an axis such as hypothalamus → anterior pituitary → target gland, and to predict the result of negative feedback, for example how TSH changes if thyroid hormone falls or if a receptor is defective.

Common confusion: Endocrine is not the same as exocrine. Exocrine glands send secretions through ducts to a surface or lumen. The pancreas has both, with islet cells endocrine and acinar cells exocrine. What settles it is the route: through the blood means endocrine, local diffusion to neighboring cells means paracrine.