Iodine metabolism and thyroid hormones

Question 1

Describe the abundance of iodine?

Answer 1

High abundance in sea

Low abundance on land

Question 2

What is the RDI for iodine?

Answer 2

150 ug

Question 3

What is iodine required for in the body?

Answer 3

Synthesis of thyroid hormones T3 and T4

Question 4

What can iodine deficiency lead to?

Answer 4

Decreased thyroid hormone synthesis > feedback to increase production of TSH > induces thyroid cell proliferation > goitre

Question 5

What is the cause of cretinism?

Answer 5

Congenital condition due to maternal iodine defciciency

Question 6

Which radioactive substance can destroy the thyroid?

Answer 6

Radiation from I-131

Question 7

Why are iodine deficiencies common in humans and land animals?

Answer 7

Absence from soils

Question 8

Describe the difference between an antiseptic and a disinfectant?

Answer 8

Antispetic: antimicrobial that reduces the risk of infection, applied externally to living surfaces

Disinfectant: antimicrobial that is applied to non-living objects

Question 9

Can iodine be used as an antibiotic, antiseptic, or disinfectant?

Answer 9

Antiseptic

Question 10

What are the advantages of using iodine as an antiseptic?

Answer 10

Wide spectrum of antimicrobial activity

When brown - can see where it is and inidicates that it is active

Question 11

Describe why iodine has an antimicrobial action?

Answer 11

Active form, I2, is a potent oxidiser

Reacts in electrophilic reactions with enzymes of respiratory chain and amino acids in cell membrane and cell wall > cell integrity destroyed

Question 12

Why is iodine used as an X-ray contrast material?

Answer 12

High atomic number

Low toxicity

Ease of attachment to organic compounds

Question 13

Is iodine a major mineral or a trace mineral in the body?

Answer 13

Trace mineral (20mg/body)

Question 14

Which substance has the highest concentration of iodine?

Answer 14

Kelp

Question 15

What is the recommended upper level of iodine intake per day?

Answer 15

1100 ug/day

Question 16

What are the most significant sources of iodine in the human diet?

Answer 16

Iodised salt

Seafood

Bread

Question 17

Briefly describe the symptoms of iodine deficiency?

Answer 17

Underactive thyroid gland

Goitre

Cretinism

Question 18

Describe the symptoms associated with iodine toxicity?

Answer 18

Underactive thyroid gland

Elevated TSH

Goitre

Question 19

What are goitrogens?

Where are they found?

Answer 19

Substances that inhibit iodide uptake in the thyroid and magnify the severity of any iodine deficiency

Found in soy, cabbage, kale, brussel sprouts

Question 20

How many people in the world are affected by iodine deficiency?

Answer 20

2 billion

Question 21

How many people are affected by goitre?

Answer 21

200 million

Question 22

How many people worldwide are affected by cretinism?

Answer 22

6 million

Most common form of mental retardation

Also most preventable

Question 23

What is the most common and also most preventable form of mental retardation?

Answer 23

Cretinism

Question 24

Describe the hypothalamic-pituitary-thyroid axis?

Answer 24

Question 25

What controls the release of thyroid hormones?

Answer 25

TSH from pituitary

Question 26

Describe the action of thyroid hormones?

Answer 26

Increase mental alertness Increase basal metabolic rate Elevate number of catecholamine receptors \> enhance catecholamine effects Stimulate differentiation and function of brown adipose tissue \> generate heat

Question 27

Why does every cell in the body rely upon thyroid hormones?

Answer 27

Required for regulation of metabolism

Question 28

Describe the proportion in which the normal thyroid produces T3 and T4?

Answer 28

80% T4 20% T3

Question 29

Describe the relative activity of T3 and T4 in the body?

Answer 29

T3 has about four times the activity of T4

Question 30

Describe the half-lives of T3 and T4?

Answer 30

T3: 6-9 days T4: 24-36 hours

Question 31

Describe the synthesis of T3 and T4?

Answer 31

Iodide oxidised by thyroperoxidase \> iodine reacts with tyrsosines in thyroglobulin \> iodinated tyrosines on surface of thyroglobulin enzymatically condensed by thyroperoxidase \> modified thyroglobulin taken up into thyroid cell \> peptide links hydrolysed \> free amino acids and T3/T4 liberated

Question 32

Where is thyroperoxidase located?

Answer 32

Embedded in thyroid cell membrane

Question 33

Describe the process of T3 and T4 secretion from the thyroid gland?

Answer 33

Question 34

Describe uptake of iodide by the thyroid gland?

Answer 34

TSH stimulates TSH receptor \> cAMP activated \> activates NIS (Na iodide symport) \> Na and I enter cell at same time

Question 35

What drives the NIS in thyroid cells?

Answer 35

Na gradient maintained by 3Na/2K ATPase

Question 36

Describe the use of radioactive iodine as treatment for goitre?

Answer 36

Iodine-131 can be used to reduce thyroid mass

Question 37

Describe the treatment options for goitre?

Answer 37

Radioactive iodine treatment Antithyroid drug therapy Surgery/thyroidectomy Thyroid arterial embolization

Question 38

How are thyroid hormones transported in the plasma? Why?

Answer 38

70-80% carried in plasma bound to thyroid binding globulin (TBG) Remainder divided between transthyretin and serum albumin Because it is lipophilic

Question 39

Describe how thyroid hormones elicit their actions?

Answer 39

Receptors bind DNA \> transcritptional repression in absence of hormone Hormone binds receptor \> transcription activated

Question 40

Compare the effects of hypothyroidism and hyperthyroidism?

Answer 40