Iodine Flashcards
role of I in thyroid metabolism
T3 = triiodothyronine (contains 3 I) = active hormone in cells
T4 = thyroxine (4 I) = transport or prohormone form in plasma
where is iodine found
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
precursors of iodine hormone
MIT and DIT
active form of I hormone
T3= triiodothyronine
transport/prohormone form of I hormone
T4= thyroxine
Iodide absorption
constant supply to replace losses (urine, sweat)
**UNLIKE MOST MICORMINS significant excretion of I (controlled) BUT no controlled absorption
where do humans store I
thyroid gland
GI absorption of I
almost 100% (not regulatory site)
absorption in stomach and small intestine
small intestine I absorption
absorb thyroid hormones as T3 and T4, so these are given directly as medication
6 steps of thyroid hormone synthesis
- I enters thyroid cell by active transport (95mcg per day taken up= EAR)
- 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 - another I added to Thg-MIT=Thg-DIT
- (IN COLLOID) Thg-DIT + Thg-DIT = (condense) Thg-T4
- Thg-DIT +Thg-MIT = (condense) Thg-T3 and Thg-rT3 *reverse
- (BACK TO THYROID CELL) T3, T4 removed/cleaved from Thg complex and released into plasma
-all unused I recycled
in thyroid hormone synthesis, when is H2O2 used
steps 2 and 4
NADPH oxidase
thyroid peroxidase and thyroperoxidase
in thyroid hormone synthesis, which reactions occur in protein Thg
step 3, 4, and 5
3. MIT + I = DIT
4. DIT + DIT = T4
5. MIT + DIT = T3
what happens in step 6 in thyroid hormone synthesis
Thg removed by degrading to amino acids
how is most T3 made
minor: condense Thg-DIT+Thg-MIT
major: in bloodstream made by deiodination of T4 (prohormone)
deiodinases
T4 = T3 or rT3 (inactive)
5’-deiodinase = T3
5-deiodinase = rT3
both require Se
control of thyroid hormone production
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
what happens if TSH stimulated during I deficiency
hyperplasia of thyroid gland “goitre”
neck swells bc more Thg is being made in anticipation of making T4
goitre common in? treatment?
developing countries, low iodine
shrink when given iodine or thyroxine
transport of T3 and T4
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
function of T3 in cells
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
function of T3 in cytosol of tissue cells
influence cell metabolism by activating phosphatidylinositol 3-kinase (PI3K) (signaling pathway)
overall function of T3
increase BMR, O2 consumption, heat production
- hypothyroidism: gain wt, fatigue, cold
CNS development
linear growth
effect of T3 on adipose tissue
enhance lipolysis
effect of T3 on muscle
enhance contraction
effect of T3 on bone
promote anabolism (growth and development)
effect of T3 on CV
increase heart rate
effects of T3 on GI
stimulate nutrient digestion and absorption
effect of T3 on metabolism
stimulate metabolic rate, oxygen consumption in metabolically active tissues
EAR I
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
RDA I
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
food sources I
seafood
dairy/eggs
IDDs
iodine deficiency disorders
goitre and cretin
both put region/country at risk for low productivity, poor quality of life
goitre
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
cretin
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
2 main causes of IDDs
- lack of I in food or water due to soil levels, lack access to seafoods
- usually mountainous areas
and/or: - 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
IDD prevalent areas globally
andes: mountains (South America)
Eastern Europe: cabbage
Nepal: Himalayas
Ethiopia: high altitude = low I in soil, don’t eat seafood
Prevention of IDDs
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
UL for I
= 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)
