Download - Parathyroid Hormone
Parathyroid Hormone
Dr. Shaikh Mujeeb AhmedAssistant ProfessorAlMaarefa College
ENDO BLOCK 412
Lecture Objectives• At the end of this lecture the student should be able to • Describe the role of calcium in the body• Enumerate the factors affecting plasma calcium• Describe the mechanisms controlling plasma Ca++ concentration• Describe the chemical nature, source and actions of parathyroid
hormone• Describe the chemical nature, source and actions of calcitonin
hormone• Describe the role of vit.D in regulation of Ca++ conc.• Describe Pathophysiology of hypo & hyperparathyroidism• Describe the effects of Vit.D deficiency
Calcium Distribution
Total Body Ca++
Body fluid (1%)
ECF (0.1%)
Free form (50%)
Bound to plasma
protein (50%)
ICF in soft tissues (0.9%)
Skeleton & teeth (99%)
Vital role of Ca++
• Neuromuscular excitability• Excitation contraction coupling in cardiac,
smooth & skeletal muscle• Stimulus secretion coupling• Excitation secretion coupling• Maintenance of tight junctions between cells• Clotting of blood
Endocrine Control of Calcium Metabolism
• Three hormones regulate plasma concentration of Ca2+ (and PO4
3-)– Parathyroid hormone (PTH)– Calcitonin– Vitamin D
Ca++ homeostasis & Ca++ balance• Ca++ homeostasis
– Ca++ adjustment on Minute to minute basis • Exchange between bone and ECF
• Ca++ balance– Slowly responding adjustment
• Ca++ intake Vs Ca++ excretion
• Parathyroid hormone (PTH) the principal regulator of Ca++ metabolism , acts directly or indirectly on all three of these effector sites.
Parathyroid hormone• secreted by the
parathyroid glands, • four rice grainsized
glands located on the back surface of the thyroid gland, one in each corner
Cont…• PTH is essential for life• The overall effect is increase plasma Ca++
• Complete absence of PTH ensues death within few days.
• It acts on bone, kidneys and intestine• PTH acts to lower plasma PO4
3- conc.
PTH raises plasma Ca2+ by withdrawing Ca2+
from the bone bank.
• induces a fast Ca2+ efflux into the plasma from the small labile pool of Ca2+ in the bone fluid.
• Second, by stimulating bone dissolution, it promotes a slow transfer into the plasma of both Ca2+ and PO4
3- from the stable pool of bone minerals in bone itself.
PTH’s chronic effect is to promote localizeddissolution of bone to release Ca2+ into plasma.
• Stimulate osteoclast• Both Ca2+ & PO4
3- are released in plasma• PTH deals with PO4 by its action on kidneys
PTH acts on the kidneys to conserve Ca2+
and eliminate PO43-
• Absorb more Ca++
• Decrease reabsorption of PO43-
• an inverse relationship exists between the plasma concentrations of Ca2+ and PO4
3-
• Activation of Vit. D• PTH indirectly promotes Ca++ & PO4
3-
absorption from the intestine by helping Vit.D
Regulation of PTH secretion• PTH secretion increases
when plasma Ca2 falls and decreases when plasma Ca2 rises.
• this relationship forms a simple negative- feedback loop for controlling PTH secretion
Calcitonin• Polypeptide hormone produced by C cells of thyroid
gland• Decreases plasma Ca ++ levels• on a short-term basis calcitonin decreases Ca ++
movement from the bone fluid into the plasma. • Second, on a long-term basis calcitonin decreases bone
resorption by inhibiting the activity of osteoclasts via the cAMP pathway.
• Calcitonin also inhibits Ca ++ and PO43- reabsorption from
the nephron
ACTIONS OF CALCITONIN ON BONE
• Decresaed osteoclastic number• Decreased osteoclastic activity• Actions are proportional to baseline rate of
bone turnover
VITAMIN D• cholecalciferol, or vitamin D, a steroid like
compound essential for Ca++ absorption in the intestine.
• Activated by the addition of two (-OH) groups.• The first reaction occurs in the liver & the
second in kidneys.• The active form of vit.D is 1,25-(OH)2- vitamin
D3 also known as calcitriol.
Fig. 19-27, p. 736
Disorder of parathyroid hormone• PTH hypersecretion – hyperparathyroidism
– Characterized by hypercalcemia & hypophosphatemia
– Clinical features: • Muscle weakness• Decrease alertness, poor memory & depression• Cardiac disturbances • Mobilization of Ca2+ & PO43- from skeletal store may
results in thinning of bone, fractures, skeletal deformities• Increase incidence of Kidney stones
Hyperparathyroidism• Bones• Stones• Abdominal groans
PTH hyposecretion• Hypoparathyroidism• Inadvertent removal of parathyroid gland
– Hypocalcemia and hyperphospatemia– Increased neuromuscular excitability– Muscle cramps twitches– Tingling and pins and needle sensation– Irritability & paranoia
SIGNS AND SYMPTOMS OF HYPOPARATHYROIDISM
• Positive Chvostek’s (facial muscle twitch) sign• Positive Trousseau’s (carpal spasm) sign• Delayed cardiac repolarization with
prolongation of the QT interval• Paresthesia• Tetany
Hypocalcemic tetany
VITAMIN D DEFICIENCY• Decrease Ca++
absorption from intestine.
• PTH increases plasma Ca++ from bone
• Demineralization of bones - softening
• Rickets in children• Osteomalacia in adult.
p. 730
1 High level of Ca2+ in bloodstimulates thyroid glandparafollicular cells to release more CT.
1 High level of Ca2+ in bloodstimulates thyroid glandparafollicular cells to release more CT.
CALCITONIN inhibitsosteoclasts, thus decreasingblood Ca2+ level.
2
1 High level of Ca2+ in bloodstimulates thyroid glandparafollicular cells to release more CT.
Low level of Ca2+ in bloodstimulates parathyroid gland chief cells to release more PTH.
CALCITONIN inhibitsosteoclasts, thus decreasingblood Ca2+ level.
3
2
1 High level of Ca2+ in bloodstimulates thyroid glandparafollicular cells to release more CT.
Low level of Ca2+ in bloodstimulates parathyroid gland chief cells to release more PTH.
CALCITONIN inhibitsosteoclasts, thus decreasingblood Ca2+ level.
PARATHYROID HORMONE (PTH)promotes release of Ca2+ frombone extracellular matrix intoblood and slows loss of Ca2+ in urine, thus increasing bloodCa2+ level.
3
4 2
1
PTH also stimulatesthe kidneys to releaseCALCITRIOL.
High level of Ca2+ in bloodstimulates thyroid glandparafollicular cells to release more CT.
Low level of Ca2+ in bloodstimulates parathyroid gland chief cells to release more PTH.
CALCITONIN inhibitsosteoclasts, thus decreasingblood Ca2+ level.
PARATHYROID HORMONE (PTH)promotes release of Ca2+ frombone extracellular matrix intoblood and slows loss of Ca2+ in urine, thus increasing bloodCa2+ level.
3
4 25
1
CALCITRIOL stimulatesincreased absorption ofCa2+ from foods, whichincreases blood Ca2+ level.
PTH also stimulatesthe kidneys to releaseCALCITRIOL.
High level of Ca2+ in bloodstimulates thyroid glandparafollicular cells to release more CT.
Low level of Ca2+ in bloodstimulates parathyroid gland chief cells to release more PTH.
CALCITONIN inhibitsosteoclasts, thus decreasingblood Ca2+ level.
PARATHYROID HORMONE (PTH)promotes release of Ca2+ frombone extracellular matrix intoblood and slows loss of Ca2+ in urine, thus increasing bloodCa2+ level.
3
4 25
6
30
References
Human physiology, Lauralee Sherwood, seventh edition.
Text book physiology by Guyton &Hall,11th edition.
Text book of physiology by Linda .S .Costanzo third edition