Thyroid Endocrinology

Question 1

Between which vertebrae is the thyroid gland found?

Answer 1

Between C5-T1

Question 2

What is the arterial supply to the thyroid gland?

Answer 2

1. Superior thyroid artery - supplies superior and anterior portions of the gland.

First branch of external carotid
2. Inferior thyroid artery - supplies postern-inferior aspect

Question 3

How is the thyroid gland innervated?

Answer 3

Innervated by branches of sympathetic trunk. However, sympathetic innervation does not control hormone release - this is controlled by the pituitary gland

Question 4

Name 2 important iodine containing molecules that the thyroid gland produces.

Answer 4

1. Thyroxine = T4 as it has 4 iodines

2. Triiodothyronine = T3 as it has 3 iodines. T3 is far more abundant.

Question 5

How is T4 converted into T3?

Answer 5

By enzymes called deiodinases

Question 6

Briefly outline the steps of thyroid hormone synthesis.

Answer 6

1. Iodide is actively cotransported across the membrane of follicular cells
2. Iodide enters the colloid of the follicle
3. In the colloid, iodide is rapidly oxidised to iodine
4. Tyrosine molecules bind to iodine on thyroglobulin molecules to produce T1 or T2
5. T1 and T2 are cleaved and join to create T3 or T4
6. Proteolysis of thyroglobulin releases T3 and T4 into the blood

Question 7

How is the thyroid stimulated to make T3 and T4?

Answer 7

Hypothalamus secretes thyroid releasing hormone (TRH) which causes the release of thyroid stimulating hormone (TSH) from the anterior pituitary gland.

Question 8

What happens if the thyroid gland is exposed to greater TSH concentrations?

Answer 8

The cells will undergo hypertrophy - aka increase in size. Results in an enlarged thyroid gland.

Question 9

What are the effects of T3?

Answer 9

1. Stimulates carbohydrate absorption from small intestine, providing energy which helps maintain metabolism at a high rate
2. Causes production of heat to maintain temperature homeostasis
3. Up regulates receptors for adrenaline. An over active thyroid will result in racing heart and anxiety
4. T3 is required for the production of growth hormone from the anterior pituitary. Absence of T3 will result in decreased growth in children.
5. T3 is important for nervous system development and for nerve & muscle reflexes