Iodine Metabolism and Thyroid Hormones

Question 1

Where does iodine have high abundance?

Answer 1

In the sea; low abundance on land

Question 2

What is the RDI for Iodine?

Answer 2

150ug

Question 3

What is iodine required to produce?

Answer 3

thyroid hormones T3 and T4

Question 4

TRH

Answer 4

thyrotropin releasing hormone

Question 5

TSH

Answer 5

thyrotropin or thyroid stimulating hormone

Question 6

How is TH release stimulated?

Answer 6

TRH from the hypothalamus releases TSH from the pituitary to act on the thyroid

Question 7

How is secretion of TRH and TSH regulated?

Answer 7

Negative feedback by thyroid hormones

Question 8

Iodine deficiency gives rise to

Answer 8

goitre

Question 9

What causes goitre?

Answer 9

High production of TSH induces thyroid cells to proliferate; occurs in iodine deficiency

Question 10

What is cretinism?

Answer 10

A congenital condition caused by maternal iodine deficiency

Question 11

What isotope of iodine can be used to destroy the thyroid?

Answer 11

I-131

Question 12

What is the main action of thyroid hormones?

Answer 12

control of metabolic rate

Question 13

How do thyroid hormones operate?

Answer 13

Via membrane receptors and direct activation of genes in DNA

Question 14

What is the microbiocidal action of iodine due to?

Answer 14

active I2 is a potent oxidizer and disrupts proton pumps and cell integrity as oxidized proteins cannot support respiration

Question 15

How much iodine is in the human body?

Answer 15

~20mg/60kg person

Question 16

How much iodine is lost every day?

Answer 16

5-10%

Question 17

What foods are high in iodine?

Answer 17

Seafood e.g. kelp, shellfish, marine fish; iodised salt; bread; dairy products; anything grown or fed from iodine-rich soil

Question 18

What are goitrogens?

Answer 18

foods that inhibit iodide uptake by the thyroid eg isoflavones (riboflavin) found in cruciferous vegetables, soy,

Question 19

The normal thyroid produces what ratio of T3 and T4?

Answer 19

80% T4; 20% T3

Question 20

How are T3 & T4 synthesized?

Answer 20

iodide is oxidised to iodine by thyroperoxidase in thyroid cell membranes; iodine reacts with tyrosines on thyroglobulin in the follicle; iodinated tyrosines on the thyroglobulin are condensed by thyroperoxidase which is taken back up into the cell; peptide linkages in the thyroglobulin are hydrolysed to release T3 and T4 into the blood

Question 21

How is iodide taken up by thyroid cells?

Answer 21

TSH binding its receptor on thyroid cells upregulates cAMP which activates the Na-I symporter (NIS) powered by the NaK-ATPase

Question 22

How are thyroid hormones carried in the blood?

Answer 22

70-80% bound to thyroid/thyroxine binding globulin (TBG); rest on transthyretin (CSF) and serum albumin

Question 23

Where is T4 deiodinated to T3?

Answer 23

liver and kidney (type I - in blood); brain, brown adipose tissue, and pituitary (type II - in cells)

Question 24

What is the normal range for T4?

Answer 24

64-154nmol

Question 25

What is the normal range for T3?

Answer 25

1.2-2.9nmol

Question 26

What is the normal range for free T4?

Answer 26

10-26pmol

Question 27

What is the normal range for free T3?

Answer 27

4-8pmol

Question 28

What is the normal range for TSH?

Answer 28

0.17-2.9mU/l

Question 29

What is the general action of T3 in the nucleus of cells?

Answer 29

Activates transcription suppressor nuclear receptor proteins to transcribe proteins and up metabolism