Parathyroid Glands: Anatomy and Physiology

What are the classic signs of Hypocalcemia?

Chvostek's sign and Trousseau's sign

Mnemonic: "Bones, Stones, Groans, Moans" What does this typically indicate?

Classical signs of hypercalcemia/ hyperparathyroidism *More common in underdeveloped countries.

What stimulates Calcitonin synthesis and secretion?

Hypercalcemia

What are the PTH effects on the intestines?

Indirect effect Calcium uptake is stimulated by the activation of Vitamin D in the kidneys.

Where is the majority of parathyroid gland vasculature originating?

Inferior thyroid arteries

Is calcitonin necessary in humans? Is it necessary for calcium homeostasis?

The body can still regulate calcium without the thyroid gland (post-thyroidectomy). Most experts don't believe Calcitonin is a major factor in calcium homeostasis.

Where is Calcitonin synthesized and what cells produce it?

Synthesized and secreted by parafollicular (c-cells) cells of the thyroid gland.

What is 1,25-dihydroxycholecalciferol?

The active form of Vitamin D.

Other than calcium, what electrolyte does PTH effect?

Phosphate

List some possible etiologies of Hypercalcemia.

- Primary hyperparathyroidism (single adenomas account for up to 89% of cases) Not endocrine related: -bone malignancies -prolonged immobilization (bone atrophy) -excess Vit D and calcium in the diet -Drug-induced (thiazides, lithium)

What are some characteristics of PTH?

- short half life of 2-3 min -major stimulus for secretion: hypocalcemia - metabolized by target tissues

What are the 3 "major players" that regulate calcium homeostasis? 8.5-10.5 mg/dL is normal

-intestines -kidneys -bones

What is the role of Calcium in ICF and ECF processes?

-muscle contraction -exocytosis -blood clotting -formation of cardiac action potentials -enzyme activation -cell signaling -bone & tooth structure

How many parathyroid glands are there?

4 typically sometimes 2 sometimes as many as 8

What are the steps between inactive Provitamin D (7-dehydrocholesterol) and active 1,25-dihydroxycholecalficerol?

7-dehydrocholecalficerol is on the skin. It becomes cholecalciferol by irradiation of the skin from the sun. Cholecalciferol enters the liver. The liver possesses the enzyme 25-hydroxylation, turning cholecalciferol into 25-hydroxycholecalciferol. This step does not require PTH. That enters the kidneys. PTH catalyzes the next step: adds another hydroxyl group (1-hydroxylation)... making it active 1,25-dihydroxycholecalciferol. The kidneys also make another inactive form of Vitamin D.

What are oxyphil cells?

A cell type in parathyroid glands. may be degenerated principal cells. Numbers increase with age. They do not make any hormones. They don't do very much...

Which of the following are possible etiologies of Hypocalcemia? A. Resection of parathyroids B. Prolonged immobilization C. Primary hypoparathyroidism D. Excess Vitamin D E. Vitamin D deficiency F. Impaired ability to activate Vit D

A. Resection of parathyroids C. Primary hypoparathyroidism E. Vitamin D deficiency F. Impaired ability to activate Vit D

PTH has direct effects on which of the following? A. Intestines B. Bone C. Kidneys

B. Bone C. Kidneys

What is the effect of PTH on phosphate?

Decreases phosphate reabsorption. More phosphate is excreted (phosphaturia) when calcium is reabsorbed.

What are the PTH effects on Kidneys?

Direct effect Stimulates the reabsorption of Calcium in distal convoluted tubules. Induces Vitamin D activation.

Calcium is very soluble in blood. True/False

False About 40% of calcium is protein-bound.

What is the major stimulus for secretion of PTH?

Hypocalcemia

What is Trousseau's sign?

Hypocalcemic tetany involving contortions of the arms and hands. AKA Carpalpedal spasm

What is Chvostek's sign?

Hypocalcemic tetany involving hyperexcitability of the facial muscles.

Why would the kidneys make 24,25-dihydroxycholecalciferol (inactive Vitamin D)?

If you already have normal calcium levels in the blood, the kidneys make inactive Vitamin D.

What hormones are synthesized by the parathyroid glands?

One hormone: Parathyroid Hormone (PTH)

What are the PTH effects on bone?

PTH directly stimulates bones. It mainly effects osteoclast activity. Bone breakdown allows for bone resorption. This liberates calcium to go into blood circulation, therefore raising serum calcium.

What is the predominant cell type in parathyroid glands? What is their function?

Principal or Chief Cells 1. synthesis and secretion of PTH 2. Sensors of minute fluctuations of ECF Calcium

What is the function of Calcitonin?

Promptly and dramatically lowers serum Calcium by acting on the bone.

What diseases are associated with Vitamin D deficiency?

Rickets occurs in children. Osteomalacia occurs in adults after normal bone growth as a child and adolescent. Bones are inadequately mineralized. Weight-bearing bones bend and deform.

What might happen if parathyroid glands cannot sense calcium levels in the blood?

They would not know when to secrete PTH. This would lead to hypoparathyroidism/hypocalcemia.

What is the importance of Vitamin D?

Vitamin D is required for calcium absorption. If it's not absorbed, it will be excreted in fecal matter.

Where are the parathyroid glands located?

posterior surface of the thyroid gland and occasionally embedded with the thyroid tissue.