Hormone axes, classic endocrine pathologies, and their hallmark lab findings.
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- What is the primary negative feedback regulator of the HPA axis?
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Cortisol (glucocorticoid) inhibits CRH release from the hypothalamus and ACTH from the pituitary.- What hormone stimulates cortisol production by the adrenal zona fasciculata?
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ACTH (adrenocorticotropic hormone) from the anterior pituitary- Which hormones comprise the HPG axis?
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GnRH (hypothalamus), FSH and LH (anterior pituitary), testosterone or estrogen/progesterone (gonads)- What is the rate-limiting enzyme in thyroid hormone synthesis?
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Thyroid peroxidase (TPO), which catalyzes iodine incorporation and coupling of tyrosine residues- TSH primarily stimulates the release of which thyroid hormones?
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T4 (thyroxine) and T3 (triiodothyronine) from thyroid follicular cells- Which thyroid hormone is more biologically active: T3 or T4?
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T3 (triiodothyronine) is 3-5 times more potent than T4- What is the classic triad of Cushing's syndrome?
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Central obesity with fat redistribution (moon facies, buffalo hump), proximal muscle weakness, purple stretch marks- In Cushing's syndrome, what is the expected 24-hour urine free cortisol compared to normal?
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Significantly elevated (normal is less than 50-100 mcg/24 hr; Cushing's typically greater than 250 mcg/24 hr)- What is the dexamethasone suppression test used for?
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The low-dose dexamethasone suppression test screens for and confirms hypercortisolism by testing whether cortisol is suppressible. Once Cushing's syndrome is confirmed, plasma ACTH (not the DST) distinguishes ACTH-dependent from ACTH-independent causes; the high-dose dexamethasone suppression test then differentiates pituitary Cushing's disease (cortisol suppresses) from ectopic ACTH secretion (cortisol does not suppress) among ACTH-dependent causes.- In Cushing's syndrome, plasma ACTH is elevated in which type(s)?
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ACTH-dependent types: pituitary adenoma (Cushing's disease) and ectopic ACTH secretion (small cell lung cancer)- What are the classic electrolyte abnormalities in primary adrenal insufficiency (Addison's disease)?
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Hyponatremia, hyperkalemia, and hypoglycemia- How does the morning plasma cortisol differ between Addison's disease and normal?
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In Addison's disease, morning cortisol is severely low (typically less than 3 mcg/dL), whereas normal is 10-20 mcg/dL- In Addison's disease, what is the expected ACTH level?
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ACTH is markedly elevated (often greater than 1000 pg/mL) due to loss of negative feedback from cortisol- What is the classic presentation of primary hypothyroidism?
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Cold intolerance, weight gain, fatigue, bradycardia, dry skin, and delayed deep tendon reflexes- In primary hypothyroidism, TSH is elevated and free T4 is low. Why does TSH rise?
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Loss of negative feedback from T4 (and T3) leads to unopposed TRH and TSH secretion
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