Iodine Flashcards
Why is iodine unique and important?
- Heaviest element required for human nutrition (AW=127)
- Responsible for just one function (synthesis of thyroid hormones). Thyroid hormones = master regulator of metabolism
- Physiological significance of the thyroid hormones are widespread. Deficiency can be fatal - still largest public health problems in the world (particularly for developing countries)
- Availability of dietary iodine is dependent on presence of iodine in soil, which differs across planet
How does the amount of iodine in soil differ and what impact does this have?
- Solubility of iodine in soil leads to variations in the bioavailability of dietary iodine
- Iodine content of soil is reduced when over-exposed to weathering (rain, snow, and glaciation) and transport back to the ocean. Because iodine is water soluble it goes back to ocean via waterways
- Iodine poor regions include large mountainous ranges and heavily flooded areas (Himalayas, Alps, Ganges river). Interior mountains
- Plants can contain up to 1.0mg/kg dry weight of iodine but can be less than <0.01mg/kg if grown in deficient areas
- Foods arising from the sea are the best natural sources of iodine
What is the significance of iodine in salt?
- the best reliable source of daily iodine is table salt (iodized
- Many developed countries that rely on processed foods have 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 make reduce iodine intake (reducing salt intake)
Name food sources and RDA of iodine
- In micrograms!!
- Most comes from seafood
How much iodine is absorbed and where is it stored in the body?
- (I-) ion is the form found in most foods and is efficiently absorbed
- 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 gland can concentrate iodine 100-fold more than plasma levels
What is the main site of iodine excretion? How much is excreted?
- Kidney is main site of excretion, normal urinary levels 50ug/g of iodine (expressed per creatine) and approx same concentration in plasma
What is the function of thyroglobulin?
- Thyroglobulin (TG) acts as the precursor to thyroid hormones (found in the colloid and secreted by follicular cells of the gland)
- Iodine is added to TG through enzyme reactions (thyroid peroxidase)
What are MITs and DITs and what can they form?
- one iodine molecule on the tyrosone = MIT (mono-iodotyrosine)
- two iodine molecules on the same tyrosine = DIT (di-iodotyrosine)
- A coupling reaction occurs that forms either T3 (DIT + MIT) or T4 (DIT + DIT) (bound to TG)
What thyroid hormones are active or non active?
- T3; tri-iodothyronine = ACTIVE form
- T4; tetra-iodothyronine (thyroxine) = inactive form
What do microsomal enzymes do? Where do they function?
- Microsomal enzymes de-ionate (activation of T3 from T4) :
- Type 1 deiodinase (liver, kidney, thyroid)
- Type 2 (brain and adipose tissue, pituitary)
- T3 is needed at different rates in these organs, it is controlled by enzymes.
Explain the structure of the thyroid
- Thyroid contains thyroid follicles. These follicles have follicular cells surrounding and colloid within that makes up the majority of the follicle
Explain the process of thyroid hormone synthesis
- Iodine comes into follicular cell via Na/K/ATPase and travels through the cell by diffusion into the colloid
- Nucleus forms TG protein which is released into colloid
- Thyroid peroxidase reaction occurs where TG and iodine form MITs and DITs which form T3 and T4
- TG stores T3 and T4 within the colloid and once hormones are needed it is released
Once T3 and T4 is formed how is it released into the blood?
- Thyroid stimulating hormone in the blood senses need for more T3 and T4
- It will bind to TSH receptor on follicular cell and cause activation of cAMP → protein kinase.
- Protein kinase phosphorylates protein leading to release of T3 and T4 bound to thyroglobulin
- Lysosome will break T3 and T4 off of TG where T3 and T4 are released into blood
What is the function of T3?
- thyroid hormones are the master regulator of metabolism
- T3 enters cells and binds to nuclear receptors that activate the synthesis of genes
- T3 is responsible for regulating:
→ Mitochondrial protein synthesis
→ Lipid metabolism
→ Carbohydrate metabolism
→ Protein metabolism
→ Ion transport
→ Muscle contraction
Explain the feedback loop of the thyroid hormones. What occurs during stressful situations or cold temperatures?
- the hypothalamus will be activated and release TRH
- TRH stimulates anterior pituitary to increase TSH secretion
- The thyroid hormone will then increase T3 and T4
- T4 will either turn off the system (decrease TRH and TSH) or be converted to T3
- Increased plama T3 will work on many tissues and the NS
- The NS will permit normal growth and development, and maintenance of normal activity
- Other tissues stimulated will increase BMR, heat production, responsiveness to sumpathetic input, and permit normal growth and development