Structure and function of hormones, hormone action, hormone receptors, and second messengers

Receptor mechanisms

Hormone receptor signalling operates through two broad mechanisms - cell surface receptors with second messenger cascades (for peptide/protein/catecholamine hormones, which cannot cross the lipid cell membrane) and intracellular/nuclear receptors (for lipid-soluble steroid and thyroid hormones, which cross the membrane directly) - this structural distinction explains differences in hormone onset speed, duration of action, and mechanism of receptor-level disease
Foundational receptor biology underlying understanding of both normal endocrine physiology and receptor-level disease mechanisms (receptor resistance syndromes, receptor-blocking autoantibodies)

Second messenger systems (e.g. cAMP via G-protein coupled receptors, or tyrosine kinase cascades for insulin/growth factor receptors) amplify and rapidly propagate the signal from a cell-surface receptor-hormone binding event - explaining the fast onset of peptide hormone effects (seconds to minutes)
Nuclear receptor binding directly alters gene transcription, explaining the slower onset (hours) but more sustained effect of steroid/thyroid hormone action

Receptor-level disease mechanisms are clinically relevant - e.g. androgen insensitivity syndrome (a normally functioning receptor cannot respond to hormone despite adequate hormone levels), and receptor-blocking or receptor-stimulating autoantibodies (e.g. TSH receptor-stimulating antibodies in Graves disease) illustrate how disease can arise at the receptor level rather than from abnormal hormone production itself
Understanding receptor-level mechanisms explains why some hormone resistance syndromes present with elevated (not low) hormone levels despite clinical features of deficiency - the gland is producing adequate or excess hormone, but the signal is not being received at the target tissue
Relevant to interpreting discordant hormone level and clinical picture combinations, prompting consideration of a receptor-level rather than production-level explanation

Androgen insensitivity syndrome (receptor resistance despite adequate hormone), Graves disease (TSH receptor-stimulating autoantibody), growth hormone insensitivity (Laron syndrome), G-protein coupled receptor signalling in endocrine disease

A discordance between measured hormone level and expected clinical effect should prompt consideration of a receptor-level (rather than production-level) explanation - conditions like androgen insensitivity syndrome and Graves disease illustrate that endocrine disease can arise from abnormal receptor function/regulation just as much as from abnormal hormone secretion itself

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