Exam 2 (Lecture 11) - Thyroid Gland

Question 1

Describe the structure of the thyroid gland, name three hormones secreted by this gland, and characterize their function.

Answer 1

1) Two lobes in most species with parathyroid glands embedded within them.
- made up of parafollicular cells and follicles consisting of follicular cells surrounding colloid

2) T3 and T4
- synthesized by tyrosine and iodide in follicular cells
- majority is T4 (with some T3)
- stored bound to thyroglobulin in colloid
- T3 is most potent and made from T4 (reverse T3 is inactive form produced by liver)
- main function is to increase basal metabolic rate (also fetal growth and development)

Calcitonin
- role in calcium homeostasis
- decreases plasma calcium level
- peptide hormone
- secreted in response to hypercalcemia or gastrin

Question 2

What is goiter?

Answer 2

Hyperplastic enlargement of the thyroid gland

Caused by iodine deficiency or Grave’s disease (autoantibodies stimulate TSH receptors of follicular cells)

Question 3

Describe the main steps in the synthesis and secretion of thyroid hormones.

Answer 3

1) Follicular cells synthesize protein thyroglobulin (TG) and also “trap” I- (against its concentration gradient using Na+/I- symporter)

2) TG and I- are released in the follicle lumen, I- is converted into highly reactive IO and this molecule iodinates tyrosine molecules within TG. If adjacent, iodinated tyrosines condense to form T3 and T4

3) TG containing T3 and T4 is taken up by phagocytosis; inside of follicular cells is attacked by proteases freeing T3 and T4

4) T3 and T4 can exit cells and enter circulation, where the circulate bound to plasma proteins

**Most of this occurs in the colloid with the follicular cell serving as kind of a bus stop for molecules

Question 4

How is the secretion of thyroid hormones regulated?

Answer 4

1) Deviations from homeostasis, such as a drop in core body temp are detected by the hypothalamus and this stimulates T3 and T4 release

2) Deviation from homeostasis activates secretion of TRH by hypothalamic neurons

3) TRH stimulates secretion of TSH by anterior pituitary

4) TSH enters circulation and stimulates the thyroid gland to release T3 and T4

5) Released thyroid hormones stimulate basal metabolic rate in many cells in the body which causes an increase of thermogenesis (an adaptation to the cold) and homeostasis is restored

Question 5

Describe the main effects of thyroid hormones on calorigenesis, on the carbohydrate and lipid metabolism, on the protein synthesis, on the bone, muscle, skin, and also on the brain development.

Answer 5

1) Calorigenesis = increased O2 consumption, BMR, heat dissipation, panting, sweating, beta-adrenergic receptors, cutaneous vasodilation and cardiac output

2) Carbohydrate metabolism = increase effects of diabetogenic hormones (GH, cortisol, glucagon, epinephrine), gluconeogenesis, glycogenolysis, intestinal carbohydrate absorption

3) Lipid metabolism = increased lipolysis, beta-oxidation, hepatic ketogenesis, hepatic triglyceride synthesis, hepatic LDL receptor synthesis, cholesterol clearance

4) Bone = maintain growth and epiphyseal closure

5) Muscle = maintain muscle protein synthesis

6) Brain = increase fetal brain development, synapse formation, and myelination

7) Skin = maintain protein synthesis and turnover; maintain hair coat and sebaceous gland activity

Question 6

In which species will you most likely encounter HYPERTHYROIDISM? What are the most common causes and physical manifestations? How is it treated?

Answer 6

1) Older cats

2) Causes:
- thyroid gland tumors
- too high or too low iodide diet

3) Symptoms:
- weak and nervous (muscle wasting)
- poor hair coat
- diarrhea
- vomiting
- goiter
- weight loss
- heat intolerant
- increased nail growth

4) Treatment:
- methimazole (inhibits T4 formation)
- surgery
- radioactive I- (takes advantage of iodide trap)

Question 7

In which species will you most likely encounter HYPOTHYROIDISM? Are there breeds which are predisposed to this endocrinopathy?

Answer 7

1) Dogs (usually primary hypothyroidism)

2) Yes:
- Golden Retriever and Doberman Pincher primarily
- also: Great Dane, Cocker Spaniel, Dachshund, Irish Setter, Shetland Sheep Dog, Airedale, Boxer, Miniature Schnauzer, Poodle, Pomeranian

Question 8

Explain the difference between the primary, secondary, and tertiary hypothyroidism. What is cretinism?

Answer 8

1) Primary (thyroid lesion):
- no increase in T4 after administering TSH

2) Secondary (pituitary lesion):
- no increase in TSH after administering TRH

3) Tertiary (hypothalamic lesion)
- increase in TSH after administering TRH

4) Cretinism = congenital hypothyroidism

Question 9

How is hypothyroidism treated?

Answer 9

Replacement therapy with synthetic thyroxine (levothyroxine)