Thyroid Hormone Synthesis and Action

Question 1

• How does the thyroid gland form?

Answer 1

Downgrowth from pharyngeal endoderm of developing tongue, C-cells from ultimobranchial body

Question 2

• What does the thyroid gland consist of?

Answer 2

Thyroid follicles – follicular cells, parafollicular cells & colloid

Question 3

• What is the function of the parafollicular cells (C-cells)?

Answer 3

Secrete calcitonin, important for calcium regulation

Question 4

• What two hormones are produced by follicular cells?

Answer 4

Triiodothyronine (T3) & thyroxine (T4) – amino acid derivatives from tyrosine

Question 5

• Describe the formation of T3 & T4, what role does iodine have to play?

Answer 5

Follicular cells synthesize thyroglobulin, iodine is required for iodination of thyroglobulin, to then produce monoiodotyrosine or diiodotyrosine, T3 = mono + di, T4 = di + di

Question 6

• How does iodine enter the follicular cells?

Answer 6

Actively transported by Na+/I- symport in the basal membrane, stimulated by TSH

Question 7

• What is stored in the colloid?

Answer 7

Primarily thyroglobulin, T3 and T4 are bound to the thyroglobulin

Question 8

• How are T3 and T4 secreted?

Answer 8

Colloid taken up into follicular cells via endocytosis, lysosomal enzymes split thyroid hormones from thyroglobulin

Question 9

• How are the majority of thyroid hormones transported in the blood?

Answer 9

More than 99% bounds to proteins e.g. thyroid-binding globulin (TBG)

Question 10

• What is an alternative source of T3?

Answer 10

T4 can be deiodinated to T3 within cell, approximately 50% of the T4 released will be

Question 11

• What happens to the remaining T4?

Answer 11

Deiodinated to reverse T3 – aim is to lower the metabolic rate

Question 12

• What is sick euthyroid syndrome?

Answer 12

Low serum T3 concentrations with normal thyroid gland function

Question 13

• What is the main site of deiodination?

Answer 13

Liver and kidneys

Question 14

• What are the main actions of thyroid hormones?

Answer 14

Maintenance of body temperature, development & growth, increases metabolic rate, increases HR and contractility, increases respiration (indirect effect), infertility with decreased levels, increases GI motility

Question 15

• How is the thyroid gland regulated?

Answer 15

Hypothalamus releases TRH (thyrotropin-releasing hormone), stimulates adenohypophysis to secrete TSH

Question 16

• Through what pathway do most anterior pituitary hormones act?

Answer 16

Adenyl cyclase pathway

Question 17

• What are the five actions of TSH

Answer 17

Increased endocytosis and proteolysis of thyroglobulin from colloid, increased activity of the Na+/I- symport, increased iodination of tyrosine, increased size and secretory activity of follicular cells, increased number of follicular cells