Week 19 Physiology

Overview of Iodine metabolism.

  • Diet –> absorbed from GIT into circulation–>  500mcg/L
Ganong's Review of Medical Physiology, 24th Edition

Ganong’s Review of Medical Physiology, 24th Edition

T3 / T4 synthesis / secretion.

  • More amount of T4 secreted than T3 but T3 is more biologically active.
  • T3 is made at site of action by deiodinating T4.
  • Reverse T3 is not biologically active
  • At interface between thyrocyte and colloid, iodide undergoes organification.
    • Iodide –> iodine
    • Incorporated into carbon 3 of thyroglobulin molecule in colloid (thyroid peroxidase on thyrocyte apical membrane)
    • The thyroid hormone sythesised remains in colloid and when required the thyrocytes endocytose part of the colloid–> lysosomal degradation–> free T4 and T3 released into circulation
    • TPO generates reactive iodine species which react with thyroglobulin–> first product is MIT monoiodotyrosine
    • MIT is then iodinated at carbon 5 –> di-iodotyrosine(DIT) (by iodotyrosine deiodinase)
    • DIT +DIT = T4 + alanine
    • MIT + DIT = T3 + alanine


Effects of T3 / T4.

  • T3 bind to receptor in nucleus (T4 binds less avidly) –> T3/receptor complex binds DNA –> transcription of cell proteins to effect functions.
  • Stimulate O2 consumption
  • Control level of metabolism
  • Help regulate lipid and carbohydrate metabolism
  • Influence body mass and Mentation
Ganong's Review of Medical Physiology, 24th Edition

Ganong’s Review of Medical Physiology, 24th Edition

Regulation of T3 / T4 secretion.

  • Throtrophin releasing hormone from hypothalamus
  • Thyroid stimulating hormone from anterior pituitary
  • T3 and T4 from thyroid which negatively feeds back to the other 2.



  • In neonatal or fetal period –> mental retardation, inhibition of growth
  • Weight gain
  • Slowing of mentation
  • Slowing of metabolism
  • Cold intolerance
  • Hair coarsening



  • Tremor
  • Hyper-reflexia
  • Increased appetite
  • Weight loss
  • Heat intollerance
  • Agitation, nervousness
  • tachycardia


Overview of Ca2+ metabolism

Ganong's Review of Medical Physiology, 24th Edition

Ganong’s Review of Medical Physiology, 24th Edition

  • Young adult has 1100g of Ca
  • 99% in skeleton
  • Plasma ca is partly bound to protein
  • Free ionized calcium in bodily fluids is the key second messenger
  • Needed in nerve function, muscle contraction and blood coagulation


Overview of bone physiology.

  • A readily exchangeable pool of 100mmol in bone
  • Stable pool of 27200mmol –> slower to mobilise
  • Osteoclasts break down bone
  • Osteoblasts lay down new osteo matrix which gets calcified.


Vitamin D and OH-Cholecalciferols.

  • Increases gut absorption of calcium
  • Increase renal  reabsorption of Ca
  • Increases activation of osteoblasts (Vit D needed for normal bone ossification) –> secondary increased activity of osteoclasts.
  • 7 dehydrocholesterol –> Vit D3 in skin under UV light(vit D3 also ingested) –> 25 hydroxycholecalciferol (in liver by p450) –> 1,25 dihydroxycholecalciferol in kidney (by 1alpha hydroxylase)= calcitriol
  • High plasma PO4 inhibits Vit D by inhibiting 1alpha hydroxylase



Ganong's Review of Medical Physiology, 24th Edition

Ganong’s Review of Medical Physiology, 24th Edition

  • Actions.
    • Increase 1alpha hydroxylase –> increase vit D
    • Increase phosphate excretion in kidney
    • Stimulate osteoblasts and osteoclasts
    • The increased renal Calcium reabsorption maybe offset by the increase in filtered calcium
  • Regulation of secretion.
    • Low plasma ionised Ca –> increased PTH release
    • Negative feedback via Ca binds G protein receptor –> phosphoinositide turnover–> inhibit PTH release
    • 1,25dihydroxycholecalciferol acts on parathyroid to decrease preproPTH mRNA
    • Increased palsma phosphate lowers plasma free Ca–> increased PTH release
    • Mg required for normal PTH secretion
    • Made as preproPTH and cleaved before secretion into plasma



  • Made by the parafollicular cells/clear cells of thyroid
  • T 1/2 10mins
  • Actions.
    • Inhibits osteoclasts –> decrease bone resorption –> Lower plasma calcium
    • Increases calcium excretion in urine
  • Regulation of secretion.
    • Secreted when plasma Ca is high
    • Secretion starts when Ca is 9.5mg/dL
  • Stimulants
    • Beta-adrenergic agonists, dopamine, oestrogens, Gastrin, cholecystokinin, glucagon, and secretin
    • gastrin being the most potent stimulus
      • plasma calcitonin level is elevated in Zollinger–Ellison syndrome and in pernicious anemia


Viva questions:

  1. Tell me about the thyroid gland and thyroid hormones.
    • T3 T4
    • Calcitonin
  2. Tell me about the metabolism of calcium.
  3. Tell me about Vitamin D and its role in calcium metabolism