Lect 27 - thyroid, parathyroids, ca2+ homeostasis

Flashcards covering thyroid and parathyroid hormones, calcium regulation, and related disorders.

What are the two main hormones secreted by the thyroid gland?

Thyroxine (T4) and triiodothyronine (T3)

What is the primary component of thyroid follicles?

Thyroglobulin (colloid)

What cells secrete calcitonin?

Parafollicular cells (C-cells)

What is the function of Calcitonin?

Lowers ECF calcium levels by reducing osteoclast activity and inhibiting calcium reabsorption by the kidney

What hormone controls thyroid hormone secretion?

Thyrotropin-releasing hormone (TRH) and Thyroid Stimulating Hormone (TSH)

Name three factors that stimulate TRH release.

Low levels of thyroid hormones, low metabolic rate, and prolonged cold exposure

What are the action of thyroid hormones?

• Increases basal metabolic rate, oxygen consumption and ATP production,

• increases activity of NA-K-ATPase,

• increases heart rate, cardiac output, and systolic BP,

• has stimulatory effects on the CNS

• neural development in fetal and infant life

Why are thyroid hormones essential during fetal and infant life?

For normal brain development and overall growth

What are the symptom of infantile hypothyroidism?

• Low metabolic rate,

• delayed growth and neural development,

• swelling around eyes and tongue

What is simple goitre?

Enlarged thyroid

What is Hashimoto’s thyroiditis?

Autoimmune disease where immune cells attack own thyroid gland, causing the thyroid to swell but produce less hormone

What is Myxedema and what can it lead to?

An edematous (swollen) appearance throughout the body which can lead to coma and death

What is Graves' disease?

Autoimmune disease where antibodies attach to TSH receptors and stimulate thyroid cells, leading to overproduction of thyroid hormones and an associated goitre

What is exophthalmos?

Swelling of tissues around/behind eyes caused by antibodies in Graves' disease

What is the role of Bones in Calcium Regulation?

Act as 'reservoir' for calcium

What percentage of calcium is stored in bones?

99%

How does short term calcium exchange between bone and ECF works?

Readily exchangeable with ECF and acts as a quick acting 'buffer' to minor short term fluctuations in calcium ion concentrations

How many parathyroid glands are there?

Usually 4 (2 superior & 2 inferior)

What cells produce parathyroid hormone (PTH)?

Chief cells/principal cells

What are Trousseau’s and Chvostek’s signs indicative of?

Hypocalcemia

What is the action of parathyroid hormone (PTH) on osteoclast activity?

PTH binds to receptors on osteoblasts, which release RANKL to stimulate osteoclast development and activity

What is Calcitriol ('active' vitamin D)?

Increases intestinal absorption of calcium and phosphate and acts on osteoblasts and osteoclasts to promote normal mineralisation of bone

How to get or activate vitamin D?

Expose skin to sun, dietary sources, converted in the liver where converted to 25-HCC, which goes to kidney where 25-HCC is converted to 1.25 DHCC (calcitriol)

What are the effect of Lack of vitamin D and/or dietary calcium?

• Inadequate calcification of new bone (Rickets)

  • abnormal epiphyseal plates in children

• abnormal mineralization of mature bone (Osteomalacia)

  • bones are weaker and prone to fractures

Location and structure of thyroid gland?

• immediately below larynx, anterior and on each side of trachea

• vascular and one of largest endocrine glands

• two lobes (right and left) with isthmus in between

synthesis and release of thyroid hormones

1. Iodide ions are actively transported from the blood into the cytoplasm of follicle cells

2. Iodide is oxidised to iodine and then binds to tyrosine residues on thyroglobulin (large protein made by cuboidal epithelial cells)

3. Iodinated tyrosine molecules link together to form T3 and T4 which stay stored in the follicle cavity attached to thyroglobulin

4. When needed, follicle cells take in thyroglobulin from the cavity via. endocytosis

5. Lysosomes break down the thyroglobulin, releasing T3, T4, and amino acids (which have been recycled)

6. T3 and T4 diffuse out of the follicle cells into the bloodstream

7. In the blood, thyroid-binding globulins (TBGs) carry T3 and T4 to target tissues

what causes infantile hypothyroidism?

• absent or poorly functioning thyroid gland

• poorly functioning pituitary

• lack of iodine in mother’s diet

calcium regulation via kidneys

kidneys can adjust the amount of calcium lose in or absorbed from urine

function of oxyphil cells

• present before puberty

• help with identification of parathyroid tissue

lack of PTH results in…?

low blood calcium

PTH is secreted in response to…?

low ECF calcium concentration

trousseau’s sign

• compression of forearm

  • produces spasm in hand and wrist

  • thumb is adducted

  • fingers bunched and wrist is flexed

chvostek’s sign

• tapping over facial nerve in front of tragus of ear

  • causes spasm of facial muscles

  • twitch of the nose or lips

PTH action on kidney

• acts on kidney to increase reabsorption of calcium and stimulate formation of calcitriol (active vitamin D)

how body gets vitamin D?

• exposure to sun, dietary sources allow body to absorb cholecalciferol

• cholecalciferol goes to liver where converted to 25-HCC

• 25-HCC goes to kidney where converted to 1.25 DHCC (calcitriol)