6 - Thyroid Hormones

Question 1

At which positions are tyrosine iodinated?

Answer 1

3 and 5

Question 2

What disease is caused by a lack of iodothyronines?

Answer 2

Cretinism

Question 3

What is the anatomy of the thyroid gland?

Answer 3

• top of trachea
• bi-lobed
• left recurrent laryngeal nerve runs close
• isthmus: bit of tissue joining 2 lobes
• pyramid: bit of tissue at top of tissue, present in some people
• comprised of follicles

Question 4

What are the thyroid follicles comprised of?

Answer 4

• follicular cells: line follicle; traps iodide using NIS and secretes T3 andT4
• colloid: proteinaceous substance found inside follicles; contains thyroglobulin
• parafollicular cells: found between follicles; provides hormones and secretes calcitonin

Question 5

What is NIS?

Answer 5

sodium-iodide symporter

Question 6

How are thyroid hormones synthesised?

Answer 6

• TSH binds to thyrotrophin receptors and stimulates iodide pump to pump iodide into cell
• TSH affects nucleus and stimulates thyroglobulin synthesis and thyroid peroxidase enzymes
• TPO converts iodide into iodine in presence of H2O2; it diffuses to colloid
• thyroglobulin (long array of amino acids with tyrosyl residues) become iodinated by iodine in one or two positions
• monoiodotyrosine and diiodotyrosine formed
• TPO allows coupling reactions of MIT and DIT to form T3 (bioactive) or T4

Question 7

What is the process of deiodination of T4?

Answer 7

• T4 main product of thyroid gland, T3 more active
• T4 deiodinated in target tissue (using deiodinases) to produce T3
• T4 deiodinated in different position forms reverse T3 (inactive)

Question 8

How are thyroid hormones released?

Answer 8

• colloid stores iodothyronines (mainly T4)
• TSH stimulates lysosomes to move towards apical membrane
• lysosomes stimulate uptake of colloid by apical membrane that fuses with it
• enzymes break down protein, liberating T3 and T4 for release into bloodstream

Question 9

How are thyroid hormones transported in the blood?

Answer 9

• 70-80% bound to thyroid-binding globulin (TBG)
• 10-15% bound to albumin and prealbumin
• some travel unbound (bioactive components)

Question 10

What are the mechanisms of action of thyroid hormones?

Answer 10

• important in foetal growth and development
• increases basal metabolic rate
• increases protein, carbohydrate and fat metabolism
• enhances effects of catecholamines
• interacts with oestrogens

Question 11

What are the genomic actions of thyroid hormones?

Answer 11

• iodothyronines enter target cell and interact with intracellular receptors
• receptor hormone complex moves to nucleus where it leads to new proteins being synthesised
• affects membrane transport pumps in brain
• stimulates metabolic activity

Question 12

What increases the production of iodothyronines?

Answer 12

• thyroid releasing hormone

- oestrogens

Question 13

What decreases the production of iodothyronines?

Answer 13

• T3/T4
• somatostatin
• glucocorticoids
• ingestion of large amounts of inorganic iodide
• thyrotrophin