The Thyroid Gland and Iodothyronines

Question 1

Describe the structure of the thyroid gland.

Answer 1

It has two lobes joined by a narrowing called the ISTHMUS

It may have a remnant of the thyroglossal duct called the PYRAMID

Question 2

Describe the cellular arrangement of the thyroid gland.

Answer 2

The cells are arranged in follicles
There are FOLLICULAR CELLS around the outside with a proteinaceous substance called COLLOID inside the follicles
There are parafollicular cells between follicles

Question 3

What three main things does TSH stimulate in the follicular cell?

Answer 3

Stimulates IODIDE CHANNELS to transport iodide from the blood into the cell at the basolateral membrane (facing the blood)
Stimulates synthesis of THYROGLOBULIN
Stimulates THYROID PEROXIDASE

Question 4

Describe the formation of T3 and T4 in the follicular cell.

Answer 4

In the presence of hydrogen peroxide, thyroid peroxidase can convert the iodide (which has been pumped into the colloid by the pendrin pumps) into a reactive iodine form.
TSH stimulates synthesis of a protein called thyroglobulin (moves to the apical surface - facing colloid) which is a long array of amino acids containing TYROSYL RESIDUES.
The tyrosyl residues can be iodinated by the reactive iodine in ONE or TWO positions forming MONOIODOTYROSINE (MIT) and DIIODOTYROSINE (DIT) respectively.
Continued stimulation of thyroid peroxidase will cause coupling reactions forming T3 and T4

Question 5

At which positions are tyrosine iodinated?

Answer 5

3 and 5

Question 6

Describe how iodothyronines are transported in the blood.

Answer 6

1. The majority of T3 and T4 are transported by THYROXINE BINDING GLOBULIN (TBG)
2. Some T3 and T4 is transported by ALBUMIN (binds more loosely)
3. Some are bound to prealbumin (very few)

The BIOACTIVE component is the T3 and T4 that is UNBOUND

Question 7

How do the latent periods and half-lives of T3 and T4 differ?

Answer 7

Latent Periods:
T3 = 12 hours 
T4 = 72 hours 
Half Lives:
T3 = 2 days 
T4 = 7-9 days

Question 8

Describe the deiodination of T4.

Answer 8

T4 can be deiodinated to produce T3 which is MORE ACTIVE

T4 can be deiodinated in a different position to produce REVERSE T3 (which is INACTIVE)

Question 9

What are the main actions of iodothyronines?

Answer 9

Increase basal metabolic rate
Increase carbohydrate, fat and protein metabolism (in both directions: anabolic and catabolic)
Enhance the effects of catecholamines
Interact with other endocrine systems
Have effect on CNS
Increase vitamin C synthesis from retinal

Question 10

Iodothyronines are important in growth and development. What disease is caused by a lack of iodothyronines?

Answer 10

Cretinism

Question 11

Describe the action of iodothyronines on target cells.

Answer 11

Thyroid hormones are NOT steroid hormones but they are LIPID SOLUBLE
They bind to intracellular receptors and have a genomic effect

Question 12

State two hormones that increase iodothyronine production.

Answer 12

Thyroid Stimulating Hormone (adenohypophysis)

Oestrogens

Question 13

State some factors that decrease iodothyronine production.

Answer 13

T3/T4 negative feedback (direct and indirect)
Somatostatin
Glucocorticoids
Ingestion of large amounts of inorganic iodide (wolf chaikoff effect)
Thyrotrophin (autonegative feedback loop)