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Endocrine system

Receptors for hormones

  1. Nuclear receptors-thyroid hormones
  2. Cytoplasmic receptors - steroidal hormones (glucocorticoids, mineralocorticoid, estrogen) vitamin D3
  3. Cell membrane receptors - Needs second messenger.

Growth Hormone:

  1. The long arm of human chromosome 17 contains gene for growth hormone and hCS.
  2. The plasma growth hormone level is less than 3 ng/mL.
  3. The half-life of circulating growth hormone in humans is 6–20 min.
  4. Daily growth hormone output is 0.2–1.0 mg/day in adults.
  5. It acts via Tyrosine Kinase using JAK-STAT pathway
  6. Reduces liver uptake of glucose, decreased insulin sensitivity (diabetogenic)
  7. Promotes gluconeogenesis in the liver
  8. Excretion of the amino acid 4-hydroxyproline is increased(collagen synthesis)
  9. GH stimulates production of somatomedin like insulin-like growth factor 1 (IGF-1) from liver

Adrenal Medulla

  1. In normal individuals 90% of output from adrenal medulla is epinephrine & only 10% is norepinephrine. Adrenal Medulla also secretes Dopamine (50%), Chromogranin A, Opioid peptides &  Adrenomedullin
  2. Extradrenal sources (sympathetic ganglia): Mainly norepinephrine & 50% of Dopamine
  3. Pheochromocytoma are catecholamine secreting tumour of chromaffin cells derived from adrenal medulla or extradrenal sources (chromaffin cells in sympathetic ganglia).Output in Pheochromocytoma  is mainly norepinephrine
  4. Norepinephrine is formed by hydroxylation and decarboxylation of tyrosine, and epinephrine by methylation of norepinephrine.
  5. Phenylethanolamine-N-methyltransferase (PNMT), catalyzes the formation of epinephrine from norepinephrine, is found in brain & adrenal medulla.
  6. Parathyroid hormone(PTH):  increases phosphate excretion in urine due to decrease in reabsorption of phosphate in the proximal tubules.
  7. PTH promotes absorption of calcium from intestine via vit.D(l,2 5 dihydroxy choleealciferol). | Calcitonin secreted by parafollicular cells or C-cells of thyroid gland. Calcitonin is stimulated when there is hypercalcemia and decreases calcium level. Calcitonin inhibits the activity of osteoclasts.    
  8. Decrease in serum calcium conc. Causes excitability of nerve and muscle cells. 
  9.  Decrease in serum sodium conc. Decrease the size of action potential-
  10. Increase in serum potassium conc. Decreases the RMP. i.e depolarisation
  11. Free ionized calcium is physiologically active form of calcium.
  12. The minimum daily iodine intake for normal thyroid function is 150 µg in adults.
  13. Iodine uptake occurs in thyroid gland, salivary gland, mammary gland, gastric mucosa, placenta, choroids plexus via Na+ /I- symporter which is a secondary active transport.
  14.  Differences between T3 and T4.






% Bound

% Free





0.15 ng/dl




T ½

Longer 6-7 days

Shorter 1-2 days




Maxm binding

TBG (67%)

Albumin (53%)



Much more rapid

In colloid

More (25%)

Less (7%)

In secretion

More (80 µg/d)

Less (4µg/d)

Reverse Form

No RT4

RT3 is present



3-5 times more potent

Binding to nuclear receptors




15. T4 and T3 are deiodinated in the liver, the kidneys, and many other tissues. One third of the circulating T4 is normally


16. Wolff–Chaikoff effect: High iodine conc. inhibiting formation of thyroid hormones due to down-regulation of sodium-iodide symporter.


17. Jod-Basedow effect: This phenomenon is an iodine-induced hyperthyroidism, typically presenting in a patient with endemic goiter

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