24.3 Biochem: Iodine metabolism and thyroid hormones

Question 1

How does a goiter occur?

Answer 1

E.g. if there is an iodine deficiency, the thyroid cannot synthesise adequate T3/T4. This reduces the negative feedback to the hypothalamus, stimulating TRH production and thus TSH production. As the thyroid gland is overstimulated, a goiter occurs.

Question 2

What are THs stored and synthesised as in the thyroid gland?

Answer 2

Iodinated thyroglobulin

Question 3

How is T4 activated?

Answer 3

By conversion to T3 by deiodinases in target cells

Question 4

How do THs exert genomic actions?

Answer 4

By binding to nuclear receptor THR which dimerises with RXR

Question 5

Where does THR/RXR undergo stimulation?

Answer 5

T3 stimulated activation in the nucleus

Question 6

What actions can THs undergo?

Answer 6

Genomic and non-genomic

Question 7

How do we regulate the bioavailability of T3?

Answer 7

With deiodinases, D1, D2 and D3

Question 8

T4 and T3:
Which is more potent?
Which is synthesised in higher quantities?

Answer 8

T3 is more potent

More T4 is produced