Lecture 13 The Thyroid Gland

Question 1

What is the function of the thyroid gland

Answer 1

Synthesises the thyroid hormones of which there are two physiologically active forms T3 (triiodothyronine) and T4 (thyroxine)

Question 2

Name the 2 cell types that the thyroid gland contain

Answer 2

C cells

Follicular cells

Question 3

What is the function of C (clear) cells

Answer 3

Secret calcitonin

Question 4

What is the function of follicular cells

Answer 4

Supports synthesis of thyroid hormones

Manufacture enzymes that make thyroid hormones as well as thyroglobulin (tyrosine residues)

Question 5

Where do the enzymes and thyroglobins get transported to from the follicular cells

Answer 5

Colloid

Question 6

What does the tyrosine residues combine with to form the thyroid hormones

Answer 6

Iodide

Question 7

How does iodide enter the follicular cells from the diet

Answer 7

Via Na+/I- transporter (symport). The coupling to Na+ enables the follicular cells to take up iodide against a concentration gradient.

Question 8

How is iodide transported into the colloid

Answer 8

Pendrin transporter

Question 9

Addition of one iodide to tyrosine

Answer 9

Monoiodotyrosine (MIT)

Question 10

Addition of two iodide to tyrosine

Answer 10

Diiodotyrosine (DIT)

Question 11

MIT + DIT

Answer 11

triiodothyronine or T3

Question 12

DIT + DIT

Answer 12

tetraiodothyronine or Thyroxine T4.

Question 13

Because T3 and T4 are lipid soluble how are they transported through the plasma

Answer 13

Bind to plasma proteins

Mainly thyroxine-binding globulin

Question 14

Thyroxine Binding Glbulin has a particularly high affinity for what hormone

Answer 14

T4- longer half life the T3

6 days compared to 1 day

Question 15

The TH receptor has a high affinity for which hormone

Answer 15

T3 3-5 times more physiologically active than T4

Question 16

Whats the effect of Thyroid Hormones

Answer 16

Raise metabolic rate
Promote thermogenesis
Increase hepatic gluconeogenesis
Increase proteolysis
Net increase in lipolysis
Stimulates GH expression (permissive)
Essential for brain development

Question 17

Name causes of hyperthyroidism

Answer 17

• Graves Disease- antibodies produced that bind mimic TSH and continually activate the thyroid gland. Increased release of TH switches off TSH release from anterior pituitary so [TSH]plasma very low. Thyroid gland may be 2-3x normal size due to hyperplasia. Hyperactivity of cells also apparent.
• Thyroid Adenoma (rare)- hormone secreting thyroid tumour

Question 18

Symptoms of hyperthyroidism

Answer 18

• Increased metabolic rate and heat production- weight loss/heat intolerance
• Increased protein catabolism- muscle weakness/weight loss
• Altered nervous system function- hyperexcitable reflexes and psychological disturbances
• Elevated cardiovascular function. TH is permissive to epinephrine, B receptors-increased HR/contractile force, high output, cardiac failure

Question 19

Causes of Hyothyroidism

Answer 19

• Hashimoto’s Disease- autoimmune attack of thyroid gland
• Deficiency in dietary iodine
Idiopathic- linked to thyroiditis

Question 20

Symptoms of Hypothyroidism

Answer 20

• Decreased metabolic rate and heat production- weight giant/cold intolerance
• Disrupted protein synthesis- brittle nails/thin skin
• Altered nervous system function- slow speech/reflexes, fatigue
• Reduced cardiovascular function- slow heart rate/weaker pulse