Iodine Flashcards
What is a unique characterisic of iodine?
It is the heaviest element required for human nutrition (AW=127)
What is iodine responsible for?
Responsible only for the synthesis of thyroid hormones
What is the physiological significance of thyroid hormones?
Widespread therefore deficiency can be be fatal
Prevalance of thyroid hormones deficiency
Still the largest public health problem in the world, particular for developing countries
What is availability of iodine dependant on?
dependent on presence of Iodine in soil, which differs across the planet.
How might iodine differ across soils?
Solubility of iodine in soil leads to variations in the bioavailability of dietary iodine since iodine is very soluble
* reduced when over-exposed to weathering (rain, snow and glaciation) and transport I back to the ocean.
* Iodine-poor regions include large mountainous ranges and heavily flooded area (Himalayas, Alps, Ganges river).
* Plants can contain up to 1.0 mg/kg dry weight of iodine but can be less than <0.01 mg/kg if grown in deficient areas.
What are the best food sources of iodine?
foods arising from the sea
What is the best reliable source of daily iodine?
iodized (fortified) table salt
How has iodized salt impacted developed countries?
Rely more on processed foods so access to iodine fortified foods (salt, dairy foods, bread making etc).
* Has been the reason for almost eradicating deficiency in these countries.
* However, some recent changes to dietary habits maybe reducing Iodine intake (reducing salt intake).
DRIs for iodine
Based on RDA
* AI for infants
Food sources of iodine
- rich sources come from the sea
- variable sources which some might depend on soil
- fortified foods
How is iodine absorbed?
Iodine rarely occurs as the element, but rather as a salt and is quickly and almost completely absorbed in the stomach and duodenum.
Storage of iodine
15-20mg iodine is present in the body at any time, stored primarily in the thyroid gland (65%).
* Rest found in kidneys and other glands.
* Thyroid can concentrate Iodine 100-fold more than plasma levels.
Excretion of iodine
Kidney is main site of excretion
* normal urinary levels 50ug/g of Iodine (expressed per creatinine) and approx. same concentration in plasma
What kind of gland is the thyroid gland?
endocrine gland
* The thyroid hormones are released directly into the blood and travel to tissues and organs all over the body
Where is iodine stored in the thyroid gland?
In the colloid of thyroid follicular cells as thyroglobulin (TG)
* Iodine goes into the cell and can be added to TG through enzyme reactions via thyroid peroxidase
How are T3 and T4 made?
A coupling reaction occurs that forms either T3 (DIT + MIT) or T4 (DIT + DIT) (bound to TG).
How is T4 converted to T3?
T4 can be converted to T3 via microsomal enzymes that de-iodinate:
* Type 1 deiodinase - liver, kidney, thyroid
* Type 2 deiodinase - brain and adipose tissue, pituitary
How are the thyroid hormones secreted from the colloid?
- Thyroid stimulating hormone (TSH) is secreted from the pituitary and travels to thyriod through circulation
- TSH binds to a G-coupled protein stimulating the second messenger system
- The active protein kinase stimulates the apical side of the follicular cell to take up the TG-T3/T4 through endocytosis
- the TG-T3/T4 is taken into cell and lysosome degrades the TG into the T3 and T4 components
- T3 and T4 are released into circulation
What is the main role of T4 besides conversion to T3?
Secretion of T4 is involved in the negative feedback system which turns off the hypothalmus and anterior pituitary from releasing TRH and TSH respectively.
* Without the negative feeedback too much TSH can be made
