Week 5. The Thyroid

Question 1

Describe the structure of the thyroid gland.

Answer 1

Sits over trachea.
Contains follicles –> epithelial follicles and colloid follicles.
Cells in between follicles - parafollicular cells. Produce calcitonin.

Question 2

What do the follicles produce?

Answer 2

Thyroglobulin - precursor for thyroid peptides

Question 3

What does thyroid releasing hormone do?

Answer 3

TRH is produced in the hypothalamus and acts on the G protein coupled receptors in the pituitary gland to release TSH (pituitary stimulating hormone).
This hormone is very small so can pass through the blood brain barrier.
TSH main regulator of the thyroid.

Question 4

How does TSH act on a cell?

Answer 4

Acts on TSH receptor on the basolateral membrane of follicular epithelial cells. Activates the cAMP pathway –> activation of enzymes in thyroid hormone synthesis

Question 5

What is the main TSH inhibitor?

Answer 5

T4

• can be converted to T3 using iodine bonding
• conversion is more effective than circulating T3

Question 6

What are thyroid hormones synthesised with?

Answer 6

• iodine and tyrosine. Tyrosine is an amino acid found in eggs, cheese and turkey, and can be synthesised using phenylalanine

Question 7

Name six key facts about iodine.

Answer 7

There’s 20mg in the body –> 80% in thyroid gland for iodothyroglobulin.
Found in muscle, salivary glands, ovaries
RDA - 150-200mg daily
Found in dairy and shellfish
Organification of iodine –> T3 and T4 production
absorbed by skin, lungs, and intestine

Question 8

What is goitre caused by?

Answer 8

Failure to regulate T3 and T4, leading to higher TSH levels.

Caused by both a deficiency or an excess in iodine.

Question 9

How are thyroid hormones transported?

Answer 9

• secreted T3/T4 are bound to protein
• only albumin bound hormones are biologically available to tissues
• T4 binds more strongly than T3 –> greater half life