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?
- Cortisol (glucocorticoid) inhibits CRH release from the hypothalamus and ACTH from the pituitary.
- What hormone stimulates cortisol production by the adrenal zona fasciculata?
- ACTH (adrenocorticotropic hormone) from the anterior pituitary
- Which hormones comprise the HPG axis?
- GnRH (hypothalamus), FSH and LH (anterior pituitary), testosterone or estrogen/progesterone (gonads)
- What is the rate-limiting enzyme in thyroid hormone synthesis?
- Thyroid peroxidase (TPO), which catalyzes iodine incorporation and coupling of tyrosine residues
- TSH primarily stimulates the release of which thyroid hormones?
- T4 (thyroxine) and T3 (triiodothyronine) from thyroid follicular cells
- Which thyroid hormone is more biologically active: T3 or T4?
- T3 (triiodothyronine) is 3-5 times more potent than T4
- What is the classic triad of Cushing's syndrome?
- 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?
- 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?
- 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)?
- 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)?
- Hyponatremia, hyperkalemia, and hypoglycemia
- How does the morning plasma cortisol differ between Addison's disease and normal?
- 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?
- 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?
- 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?
- Loss of negative feedback from T4 (and T3) leads to unopposed TRH and TSH secretion