Iodine

Question 1

role of I in thyroid metabolism

Answer 1

T3 = triiodothyronine (contains 3 I) = active hormone in cells
T4 = thyroxine (4 I) = transport or prohormone form in plasma

Question 2

where is iodine found

Answer 2

oceans and deposited in soil
plant sources= amount depends on soil
animal sources= amount depends on plants eaten, water, how and were animals were fed
SEAFOODS= rich source

Question 3

precursors of iodine hormone

Answer 3

MIT and DIT

Question 4

active form of I hormone

Answer 4

T3= triiodothyronine

Question 5

transport/prohormone form of I hormone

Answer 5

T4= thyroxine

Question 6

Iodide absorption

Answer 6

constant supply to replace losses (urine, sweat)

**UNLIKE MOST MICORMINS significant excretion of I (controlled) BUT no controlled absorption

Question 7

where do humans store I

Answer 7

thyroid gland

Question 8

GI absorption of I

Answer 8

almost 100% (not regulatory site)
absorption in stomach and small intestine

Question 9

small intestine I absorption

Answer 9

absorb thyroid hormones as T3 and T4, so these are given directly as medication

Question 10

6 steps of thyroid hormone synthesis

Answer 10

1. I enters thyroid cell by active transport (95mcg per day taken up= EAR)
2. I added to thyroglobulin (Thg) at a a tyrosine -> monoiodotyrosine (MIT)
   - Fe-dependent thyroid peroxidase
   - complex is called Thg-MIT
   - Thg is protein made in thyroid cell
3. another I added to Thg-MIT=Thg-DIT
4. (IN COLLOID) Thg-DIT + Thg-DIT = (condense) Thg-T4
5. Thg-DIT +Thg-MIT = (condense) Thg-T3 and Thg-rT3 *reverse
6. (BACK TO THYROID CELL) T3, T4 removed/cleaved from Thg complex and released into plasma
   -all unused I recycled

Question 11

in thyroid hormone synthesis, when is H2O2 used

Answer 11

steps 2 and 4
NADPH oxidase
thyroid peroxidase and thyroperoxidase

Question 12

in thyroid hormone synthesis, which reactions occur in protein Thg

Answer 12

step 3, 4, and 5
3. MIT + I = DIT
4. DIT + DIT = T4
5. MIT + DIT = T3

Question 13

what happens in step 6 in thyroid hormone synthesis

Answer 13

Thg removed by degrading to amino acids

Question 14

how is most T3 made

Answer 14

minor: condense Thg-DIT+Thg-MIT
major: in bloodstream made by deiodination of T4 (prohormone)

Question 15

deiodinases

Answer 15

T4 = T3 or rT3 (inactive)
5’-deiodinase = T3
5-deiodinase = rT3
both require Se

Question 16

control of thyroid hormone production

Answer 16

pituitary gland releases thyroid stimulating hormone (TSH)-thyrotropin
-> promotes I uptake and protein synthesis by the thyroid gland
low T3 = more TSH = more I into gland
high T3= less TSH = prevent further I uptake

Question 17

what happens if TSH stimulated during I deficiency

Answer 17

hyperplasia of thyroid gland “goitre”
neck swells bc more Thg is being made in anticipation of making T4

Question 18

goitre common in? treatment?

Answer 18

developing countries, low iodine
shrink when given iodine or thyroxine

Question 19

transport of T3 and T4

Answer 19

most plasma thyroid hormone is T4
both bind to transport proteins
T4= thyroxine binding globulin
T3= albumin
transthyretin- cotransports with RBC/retinol
small amount T3 and T4 unbound
bind receptors and enter cells to exert hormone actions

Question 20

function of T3 in cells

Answer 20

in cells T4 (5’deoidonase) = T3
in cells of tissues (liver, brain, pituitary, brown adipose, muscle)
- T3 binds nuclear receptors = influence transcription by increase mRNA selected genes

Question 21

function of T3 in cytosol of tissue cells

Answer 21

influence cell metabolism by activating phosphatidylinositol 3-kinase (PI3K) (signaling pathway)

Question 22

overall function of T3

Answer 22

increase BMR, O2 consumption, heat production
- hypothyroidism: gain wt, fatigue, cold

CNS development

linear growth

Question 23

effect of T3 on adipose tissue

Answer 23

enhance lipolysis

Question 24

effect of T3 on muscle

Answer 24

enhance contraction

Question 25

effect of T3 on bone

Answer 25

promote anabolism (growth and development)

Question 26

effect of T3 on CV

Answer 26

increase heart rate

Question 27

effects of T3 on GI

Answer 27

stimulate nutrient digestion and absorption

Question 28

effect of T3 on metabolism

Answer 28

stimulate metabolic rate, oxygen consumption in metabolically active tissues

Question 29

EAR I

Answer 29

EAR set to meet requirements for uptake by thyroid gland= 95mcg/day maintain balance of I through reuse/recycled I and replace losses with diet

Question 30

RDA I

Answer 30

150mcg 2SD = 20% pregnancy = higher 220mcg lactation = higher 290mcg thyroidectomy (removed by surgery, or ablation with radioactive I) if complete: prescribe T3/T4 and dietary I unnecessary if incomplete: RDA would supply sufficient for capacity of gland, supplementation may or may not be necessary

Question 31

food sources I

Answer 31

seafood dairy/eggs

Question 32

IDDs

Answer 32

iodine deficiency disorders goitre and cretin both put region/country at risk for low productivity, poor quality of life

Question 33

goitre

Answer 33

attempt by thyroid gland to synthesize thyroid hormones = overstimulated by TSH hyperplasia of gland (increase number of cells in gland) if pop prevalence exceeds 10%= endemic goitre symptoms: hypothyroidism = low BMR = wt gain, poor growth in children bc can't produce enough hormone

Question 34

cretin

Answer 34

child of mother deficient during pregnancy hypothyroidism in the fetus/young children prevents CNS development = - permanent mental retardation - neurological effects - growth abnormalities known as cretinism

Question 35

2 main causes of IDDs

Answer 35

1. lack of I in food or water due to soil levels, lack access to seafoods - usually mountainous areas and/or: 2. consumption of goitrogens = foods that inhibit iodine metabolism ex) generate SCN- (thiocyanate) which interferes with uptake of I by thyroid gland foods of cabbage species (eastern europe), some types of cassava (philippines, africa) *diff countries have diff I medications, Can only T4 bc soil levels/ food levels are high ALSO: when Se (need for T3->T4), Vit A, and/or Fe low, then problem is worsened for I deficiency

Question 36

IDD prevalent areas globally

Answer 36

andes: mountains (South America) Eastern Europe: cabbage Nepal: Himalayas Ethiopia: high altitude = low I in soil, don't eat seafood

Question 37

Prevention of IDDs

Answer 37

fortification of a staple (salt or sugar) with KI, iodization is effective in Can, mandatory to add 76mcg/g table salt (2g meets RDA) *none in pickling salt, sea salt, fleur de sel, Himalayan pink salt As ppl use less table salt, I intake is falling in New Zealand manufacturers now required to use iodized salt in making processed foods

Question 38

UL for I

Answer 38

= 1100mcg all sources LOAEL= 1700mcg, UF= 1.5 UL based on having elevated serum TSH and goitre-like symptoms (hypothyroidism) with too much I *paradoxical, hit, unexplained wt loss, hyperthyroid Norhtern Japan- high consumption of seafoods and seaweeds leads to I toxicity (dried seaweed = 3000mcg/g or higher)