Endocrine Physiology - The Thyroid Gland

Question 1

The thyroid contains 2 cell types:

Answer 1

• C (clear) cells whicih secrete calctonin (Ca2+ regulatory hormone)
• Follicular cells which support thyroid hormone synthesis and surround hollow follicles

Question 2

What are thyroid follicles?

Answer 2

Sphericle structures whose walls are made up of follicular cells. The centre of a follicle is filled with colloid = sticky glycoprotein matrix. Contains 2 to 3 months suppply of TH.

Question 3

Which of T3 and T4 is more pyhsiologically active?

Answer 3

The TH receptor has a much higher affinity for T3 than T4 making T3 3-5 times more physiologically active than T4.

Question 4

How is T4 converted to T3?

Answer 4

T4 is deiodinated to T3 by deiodinase enzymes. Around half the T4 is deiodinated in plasma, the remaining fraction being deiodinated inside target cells.

Question 5

Function of thyroid hormone.

Answer 5

Raises metabolic rate and promotes thermogenesis, typically through promoting futile cyles of simultaneous catabolism and anabolism.

• increases hepatic gluconeogenesis (although no effect on BG proving pancreas is releasing adequate insulin)
• net increase in proteolysis
• net increase in lipolysis
• critical for growth - stimulates GH receptor expression
• essential for brain development in utero

Question 6

Regulation of TH release.

Answer 6

Glucocorticoids and somatostatin (GHIH) are inhibitory:

• GC inhibit TSh and conversion of T4 to T3
• SS inhibits TSH (TH requried for GH action)

Question 7

Causes of hyperthyroidism

Answer 7

• Graves disease
• Thyroid adenoma (rare) - hormone secreting thyroid tumour

Question 8

Pathopyhsiology of Graves disease.

Answer 8

Antibodies are produced that mimic TSH and continually acticate the thyroid gland. Increased release of TH switches off TSH release from pituitary so [TSH] plasma very low.

Question 9

Result of increased metabolic rate and heat production in hyperthyroidism.

Answer 9

Weight loss / heat intolerance.

Question 10

Result of increased protein catabolism in hyperthyroidism.

Answer 10

Muscle weakness / weight loss

Question 11

Result of altered nervous system function in hyperthyroidism.

Answer 11

Hyperexcitable reflexes and psychological disturbances

Question 12

Result of elevated cardiovascluar function (TH is permissive to epinephrine, B receptors) in hyperthyroidism.

Answer 12

Increased HR / contractile force, high output, cardiac failure.

Question 13

Causes of hypothyroidism.

Answer 13

• Hasimoto’s disease - autoimmune attach of thyroid gland
• Deficiency in dietry iodine
• Idiopathic - may be linked to thyroiditis

Question 14

Result of decreased metabolic rate and heat production in hypothroidism.

Answer 14

Weight gain / cold intolerance.

Question 15

Result of disrupted protein synthesis in hypothroidism.

Answer 15

Brittle nails / thin hair

Question 16

Result of altered nervous system function in hypothroidism.

Answer 16

Slow speech / reflexes, fatigue

Question 17

Result of reduced cardiovascular function in hypothroidism.

Answer 17

Slow heart rate / weaker pulse

Question 18

Pathopyhsiology of a goitre in thyroid disease.

Answer 18

Goitre formation may be caused by increased tropic action of TSH on thryoid follicular cells (hypothyroidism) or over activity as a result of autoimmine disease (Graves Disease) - resulting in hypertrophy of thyroid gland.