Anatomy and physiology of the endocrine system

The endocrine system

The endocrine system comprises glands (pituitary, thyroid, parathyroid, adrenal, pancreas, gonads) secreting hormones into the bloodstream to regulate distant target organs - organised hierarchically for several axes (hypothalamus -> pituitary -> peripheral gland) with negative feedback loops maintaining hormone levels within a physiological range
Foundational anatomical and physiological knowledge underlying essentially all endocrine disease understanding and investigation interpretation

Negative feedback is the dominant regulatory mechanism across most endocrine axes - the peripheral hormone (e.g. cortisol, thyroid hormone, sex hormones) suppresses upstream stimulating hormone secretion (ACTH/CRH, TSH/TRH, LH-FSH/GnRH respectively) once adequate levels are reached, maintaining homeostasis - this explains why measuring both the peripheral hormone and its upstream regulator together (rather than either alone) is needed to correctly localise a disturbance to central or peripheral origin
Hormone action mechanisms differ by hormone class - peptide/protein hormones act via cell surface receptors and second messenger systems (rapid onset), while steroid/thyroid hormones cross the cell membrane to act on intracellular/nuclear receptors affecting gene transcription (slower onset, more sustained effect)

Understanding the specific axis anatomy and feedback relationship for a given hormone directly informs correct test interpretation (see for a worked example) and correct localisation of a suspected abnormality
Correct axis-level understanding underlies rational choice between basal and dynamic testing, and between central and peripheral gland-directed treatment

Hypothalamic-pituitary-adrenal axis, hypothalamic-pituitary-thyroid axis, hypothalamic-pituitary-gonadal axis, peptide vs steroid hormone receptor mechanism distinction

Negative feedback regulation is the organising principle across nearly every endocrine axis - understanding where in a specific axis a disturbance sits (central regulator vs peripheral target gland) requires measuring both levels together, which is the single most useful general interpretive principle across endocrine test results

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