THE THE PARATHYROID PARATHYROID GLANDGLAND

Parathyroid Gland

• 4 small glands located on the dorsal side of the thyroid gland

• essential for life• produces parathyroid hormone• responsible for monitoring

plasma Ca2+

Anatomy & Location of Parathyroid Gland

PARATHYROID

HORMONE

Parathyroid Hormone (PTH)

• a peptide hormone that increases plasma Ca2+

• causes increase in plasma Ca2+ by:

1. Mobilization of Ca2+ from bone2. Enhancing renal reabsorption3. Increasing intestinal absorption

(indirect)

Regulation of PTH release by Plasma Ca2+ Levels

• PTH is released by chief cells in the parathyroid gland.

• Chief cells contain receptors for Ca2+

• A decrease in plasma Ca2+ levels mediates the release of PTH

• Conversely, hypercalcemia inhibits PTH release

Physiologic Anatomy of the Parathyroid Gland

Effect of Parathyroid Hormone on Calcium and Phosphate Concentrations in the ECF

The rise in calcium concentration is caused by two Effects

1. an effect of PTH to increase calcium and phosphate absorption from the bone

2. a rapid effect of PTH to decrease the excretion of calcium by the kidneys

- Parathyroid Hormone Increased Calcium and Phosphate Absorption from the Bone

- Rapid Phase of Calcium and Phosphate Absorption – Osteolysis

- Slow Phase of Bone Absorption and Calcium Phosphate Release – Activation of the Osteoclasts

- Parathyroid Hormone Decreases Calcium Excretion and Increase Phosphate Excretion in the Kidneys

- Parathyroid Hormone Increases Intestinal absorptionof Calcium and Phosphate

- Cyclic Adenosine Monophosphate Mediates the Effects of Parathyroid Hormone

ACTIONS OF PTH ON TARGET ORGANS

1. BONES OR SKELETON Calcium and Phosphate absorption from bones

a. Rapid Phase – osteolysisb. Slow Phase – activation of osteoclasts

1. immediate activation of osteoclasts that are

already formed2. formation of new osteoclasts from

osteoprogenator cells 2. INTESTINES

Enhances both Ca and PO4 absorption from the intestines by increasing formation of 1, 25 dihydroxycholecalciferol from vitamin

3. KIDNEYS a. Increased renal tubular reabsorption of calcium in the distal tubules and collecting ductsb. diminished proximal tubular reabsorption of PO4

EFFECTS OF PTH ON TARGET ORGANS

1. Blood Calcium and Phosphatea. Hypercalcemiab. Hypophosphatemia

2. Urinea. Hypocalciuriab. Hyperphosphatemia - phosphaturic

Control of Parathyroid Hormone Secretion

CALCITONIN

Peptide hormone secreted by the thyroid gland that tends to decrease plasma calcium concentration.

Two ways:

1. decrease absorptive activities of the osteoclasts and the osteocytic effect of the osteocytic

membrane – immediate effect

2. decrease formation of new osteoclasts – more prolonged effect

Increased Plasma Calcium Concentration Stimulates Calcitonin Secretion

Calcitonin Has a Weak Effect on Plasma Calcium Concentration in Adult Human

SUMMARY OF CONTROL OF CALCIUM ION CONCENTRATION

1. Buffer Function of the Exchangeable Calcium in Bones - the First Line of Defense

2. Hormonal Control of Calcium ion concentration – the Second Line of Defense

a. PTHb. Calcitonin

PARATHYROID HORMONE and BONE DISEASE

1. Hypoparathryroidisma.Tetany – Calcium concentration is 6-7

mg% - Larygospasm

Signs of Latent Tetany1. Chvostek’s sign2. Trousseau’s sign

b. Impairment of blood clotting

2. Hyperparathyroidisma. Bone Disease- broken bones- Cystic bone disease – Osteitis Fibrosa Cystica or Von Recklinghausen’s Disease

b. Effects of Hypercalcemia 1. Rise of calcium to 12-15 mg% - depression of central and peripheral nervous

system - muscular weakness, constipation, abdominal

pain, peptic ulcer, lack of appetite, depressed relaxation of the heart during diastole

2. Formation of kidney stones

3. Parathyroid hormone poisoning and metastatic calcification- 17 mg% Calcium

•alveoli of the lungs – tubules of kidneys•thyroid gland – walls of the arteries•stomach mucosa

BONE DISEASES

Rickets – Vitamin D Deficiency

- occurs mainly in children

- results from calcium or phosphate deficiency in the ECF caused by lack of VitaminD

- develops tetany

Osteomalacia – “Adult Rickets” - deficiency of both vitamin D and calcium occurs as aresult of steatorrhea (failure to absorb fat) because vitamin D is fat soluble

- “Renal rickets” is a type of osteomalacia that results from kidney damage

Osteoporosis – Decreased Bone Matrix

- the most common of all bone disease in adults especially in old age

- results from diminished bone matrix rather than poor bone calcification

Causes:a. lack of physical stress on the bones due to

inactivityb. malnutrition – sufficient protein matrix

cannot be formedc. lack of vitamin C – necessary for secretion

of intercellular substancesd. postmenopausal lack of estrogen secretion

– estrogen have osteoblast-stimulating activitye. old age – diminished growth hormonef. Cushing’s syndrome – massive glucocorticoids cause decreased deposition of protein thus depressing osteoblastic activity