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Lecture 1

NUTR 3210 Lecture Notes - Lecture 1: Retinyl Palmitate, Vitamin A Deficiency, Thyroid Hormone Receptor

16 pages38 viewsFall 2016

Department
Nutrition
Course Code
NUTR 3210
Professor
Monk
Lecture
1

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Vitamins and Minerals: Part 1
Iodine/Iodide
The mineral iodine exists as I2 (elemental iodine) or as the anion iodide (I-),
iodide is the form we find in our diet
A lot of geographical areas are deficient in iodine, with close to 30% of the
worlds population estimated to live in areas of deficiency; rain washes iodide
out of most inland soils
Iodide is very water soluble, so it is found concentrated in the ocean,
consequently, seafood is a good dietary source, and intake is higher in coastal
populations
People in many countries, even in geographical deficient areas, are exposed to
adequate iodine in the diet b/c of iodide fortification of salt; however, even
some developed countries are surprisingly devoid of fortification programs
Iodine & Thyroid Hormones
The only role of iodide in the body is to participate in the synthesis of thyroid
hormones
Thyroid hormones include T3 (triiodothyronine) and T4 (thyroxin)
T3 is the bioactive form of the hormone and can be made from T4;
T is atuall osidered the prohoroe, or preursor horoe to T
With good iodide status, the ratio of T4:T3 in the blood is 20:1 and the T4
pools acts as a reserve for T3 formation
T3 binds to a steroid hormone receptor called thyroid hormone receptor (THR)
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Iodide Digestion, Absorption & Digestion
Dietary iodide will be well absorbed across the small instestine and to the blood it travels by
active transport through to the thyroid gland
T3/THR complex will function as a transcription factor and influence gene expression
How does Iodide Make Thyroid Hormones?**
Once in the thyroid, iodide reacts with hydrogen peroxide to forma radical (I-), a reactive iodide
species
It attacks to the tyrosine ring, and iodinized thyroglobulins cross link together
Enzymes then cleave to cross linked protein to release the smaller thyroid hormone molecules
Both T4 and T3 can be made this way, although the vast majority of T3 is formed from T4 in the
liver
this ia very unique situation, in which there is a chemical production of a hormone that
takes plae o the surfae of a protei, as a post-traslatioal odifiatio
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Thyroid Hormone Signaling
1. T3 is lipophilic and crosses the plasma membrane
- In the nucleus it binds to THR and forms a T3/THR complex and acts as a transcription factor to
increase gene expression
- Free THR decreases transcription of downstream genes
2. T3/THR binds to the promoter region of specific regions
3. Gene regulated by T3 signaling (ie gene expression is enhanced by high blood T3 levels)
- Growth hormone synthesis
- Na+-ATPase and Ca2+ ATPase (maintain ionic gradient for cell signaling)
Health Consequences
If dietary and blood iodide are too low, the thyroid is still unable to produce
sufficient T3 and T4
- The pituitary gland further increases blood TSH levels
Excessive stimulation of the thyroid gland results in hyperplasia (new cells)
and hypertrophy (bigger cells), causing the thyroid to enlarge this is called
goiter
Mild goiters can be reversed, but long term it may result in fibrosis and
utltimately cancer, with a poor prognosis for survival
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