Iodine Metabolism and Thyroid Hormones

Question 1

What is the recommended daily intake for iodine?

Answer 1

150µg/day

Question 2

What does iodine deficiency give rise to?

Answer 2

Goitre, when high production of TSH induces thyroid cells to proliferate.

Question 3

What is cretinism?

Answer 3

A congenital condition caused by maternal iodine deficiency

Question 4

What therapy can be used to destroy an overactive thyroid?

Answer 4

Radiation from ¹³¹I^-

Question 5

What are thyroid hormones?

Answer 5

• Master controller of metabolic rate.
• They operate via membrane receptors and direct activation of genes in DNA.

Question 6

What is the advantage of iodine antiseptics?

Answer 6

They have a wide scope of antimicrobial activity, killing all principal pathogens and even spores.

Question 7

What is the microbial action of iodine due to?

Answer 7

• I₂ is a potent oxidizer.
• It reacts in electrophilic reactions with enzymes of the respiratory chain as well as with amino acids located in cell membrane and cell wall proteins.
• It also inactivates the proton pump.

Question 8

Why is organic iodine not toxic?

Answer 8

Because it does not dissociate to free I₂.

Question 9

What is goitre caused by?

Answer 9

A combination of iodine deficiency and goitrogens

Question 10

What do goitrogens do?

Answer 10

Inhibit iodide uptake in the thyroid and magnify the severity of any iodine deficiency.

Question 11

Where are goitrogens found?

Answer 11

In soy, cabbage, kale, brussel sprouts and other cuciferous vegetables in the genus Brassica.

Question 12

What are cretins?

Answer 12

Children born from iodine deficient mothers, causing goitre, mental retardation and deafness, all of which are irreversible.

Question 13

What does thyroid hormone alter?

Answer 13

1. Energy level. Elevates ATP, acetyl-CoA, NADH and NADPH.
2. Synthesis of enzymes.
3. Increase mental alertness and BMR - elevate the number of catecholamine receptors, enhance catecholamine effects and stimulate differentiation and function of brown adipose tissue to generate heat.

Question 14

What does the normal thyroid produce?

Answer 14

80% T₄ and 20% T₃.

Question 15

How are T₃ and T₄ synthesised?

Answer 15

Peroxidase convertes 2I^- to I₂, which is then attached to tyrosine in thyroglobulin to make MIT or DIT, which then undergo condensation to make T₃ or T₄. See image.

Question 16

What catalyses the condenstion of DIT+DIT to T₃ or T₄?

Answer 16

Thyroperoxidase.

Question 17

What is the half-life of T₄?

Answer 17

6-9 days

Question 18

What is the half-life of T₃?

Answer 18

24-36 hours

Question 19

What is reverse T₃?

Answer 19

Inactive T₃

Question 20

What is the use of reverse T₃?

Answer 20

Dumping of excess iodine as an inactive product.

Question 21

What converts T₄ to T₃?

Answer 21

Di-iodinase

Question 22

How are T₃ and T₄ released into the bloodstream?

Answer 22

See image

Question 23

How is iodide taken up by the thyroid?

Answer 23

By the NIS symporter.

Question 24

What drives the NIS symporter?

Answer 24

A Na⁺ gradient maintained by a 3Na⁺/2K⁺ ATPase.

Question 25

What is Grave's disease also known as?

Answer 25

Hyperthyroidism

Question 26

What are the treatment options for Grave's disease?

Answer 26

1. Radioactive iodide 2. Antithyroid drug therapy (ATD) 3. Surgery/Thyroidectomy 4. Thyroid arterial embolization

Question 27

Which drugs are used in antithyroid drug therapy?

Answer 27

Propylthiouracil and methimazole .

Question 28

What is the action of antithyroid drug therapy?

Answer 28

Inhibition of thyroperoxidase.

Question 29

How is the majority of T₃ and T₄ carried in the plasma?

Answer 29

Bound to thyroid binding globulin (TBG). The remainder is divided between transthyretin and serum albumin.

Question 30

Which is more abundant, plasma T₃ or T₄?

Answer 30

T₄

Question 31

What is most T₃ in the circulation created by?

Answer 31

* Type I deiodinase activity in the liver and kidney. * Type II deiodinases in the brain, brown adipose tissue and pituitary produce T₃ within cells.

Question 32

What is the function of the thyroid hormone receptor in the absence of thyroid hormone?

Answer 32

Binds DNA, leading to transcriptional repression.

Question 33

What happens to the thyroid hormone receptor when thyroid hormone binds?

Answer 33

Undergoes conformation change in the nuclear receptor that causes it to function as a transcriptional activator.

Question 34

What is the function of the thyroid hormone receptor in the presence of thyroid hormone?

Answer 34

* Thyroid hormone sets the rate of synthesis of many proteins. * The proteins of oxidative phosphorylation are upregulated so more ATP is made.

Question 35

What are symptoms of hyperthyroidism (Grave's disease)?

Answer 35

* Difficulty sleeping * Heat intolerance * Infertility * Irritability * Muscle weakness * Nervousness * Scant menstrual periods.

Question 36

What are the symptoms of hypothyroidism (Hashimoto's disease)?

Answer 36

* Arthritis * Cold intolerance * Depression * Dry skin * Fatigue * Forgetfulness * Heavy menstrual periods * Infertility * Muscle aches