Endocrinology 6 - Thyroid Hormones

Question 1

Describe the structure of the thyroid

Answer 1

• There are two lobes, which contain follicular cells surrounding colloid and parafollicular cells.
• Parathyroid glands are above and below the thyroid

Question 2

What do parafollicular cells synthesise?

Answer 2

Calcitonin

Question 3

What is the role of the parathyroid gland?

Answer 3

Control calcium ion levels

Question 4

Describe the process that happens in the follicular cells following TSH binding.

Answer 4

• Iodine ions in the blood are taken up by the parafollicular cells due to a sodium iodide symporter.
• TSH acts as a transcription factor via a G-Protein coupled receptor
• This causes production of thyroglobulin, which is released into the colloid
• TSH causes thyroperoxidase to migrate to the colloid and take part in iodination and the coupling reaction.
• TSH causes lysozyme to fuse with phagocytosed products of colloid and release T3 and T4

Question 5

What happens in iodination?

Answer 5

• Iodide travels to the apical membrane and through an ion transporter.
• Thyroperoxidase and hydrogen peroxide catalyses the addition of iodine to the thyroglobulin.
• This produces mono-iodotyrosine and diiodotyrosine

Question 6

What happens in coupling reactions?

Answer 6

• Monoiodotyrosine and diioidotyosine combine to form T3 or T4

Question 7

What is T3?

Answer 7

Tri-iodothyronine

Question 8

What is T4?

Answer 8

Tetra-iodothyronine/thyroxine

Question 9

How is T3 and T4 transported around the body?

Answer 9

Mostly bond to plasma proteins

• Thyroid binding globulin (75%)
• Albumin
• Prealbumin

Question 10

What percentage of T4 and T3 are unbound?

Answer 10

• 0.05% T4

- 0.5% T3

Question 11

What are the latent periods of T3 and T4?

Answer 11

T3 12 hours

T4 72 hours

Question 12

What are the half lives of T3 and T4?

Answer 12

T3 2 days

T4 7-9 days

Question 13

Why does deiodination of thyroxine occur?

Answer 13

• T4 is the main product of the thyroid gland, but T3 is biologically active.
• Can also be deiodinated to produce reverse T3 (biologically inactive)

Question 14

What are the main actions of iodothyronines?

Answer 14

• Increased metabolic rate
• Increased protein, carbohydrate and fat metabolism
• Potentiate the actions of catecholamines (causing tachycardia)
• Interact with other endocrine systems
• Effect the CNS
• Increase vitamin A synthesis

Question 15

What are the three mechanisms of action of iodothyronines?

Answer 15

• Stimulate membrane transport mechanisms
• Stimulate metabolic activity
• Increase protein synthesis

Question 16

Describe how negative feedback controls TSH production

Answer 16

• When levels of TSH increase, this inhibits levels of TRH.
• Iodide decreases the thyroid response to TSH
• Iodine also inhibits thyroid hormone secretion