Iodine Flashcards

1
Q

Why is iodine unique and important?

A
  • 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
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2
Q

How does the amount of iodine in soil differ and what impact does this have?

A
  • 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
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3
Q

What is the significance of iodine in salt?

A
  • 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)
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4
Q

Name food sources and RDA of iodine

A
  • In micrograms!!
  • Most comes from seafood
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5
Q

How much iodine is absorbed and where is it stored in the body?

A
  • (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
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6
Q

What is the main site of iodine excretion? How much is excreted?

A
  • Kidney is main site of excretion, normal urinary levels 50ug/g of iodine (expressed per creatine) and approx same concentration in plasma
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7
Q

What is the function of thyroglobulin?

A
  • 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)
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8
Q

What are MITs and DITs and what can they form?

A
  • 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)
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9
Q

What thyroid hormones are active or non active?

A
  • T3; tri-iodothyronine = ACTIVE form
  • T4; tetra-iodothyronine (thyroxine) = inactive form
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10
Q

What do microsomal enzymes do? Where do they function?

A
  • 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.
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11
Q

Explain the structure of the thyroid

A
  • Thyroid contains thyroid follicles. These follicles have follicular cells surrounding and colloid within that makes up the majority of the follicle
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12
Q

Explain the process of thyroid hormone synthesis

A
  • 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
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13
Q

Once T3 and T4 is formed how is it released into the blood?

A
  • 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
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14
Q

What is the function of T3?

A
  • 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
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15
Q

Explain the feedback loop of the thyroid hormones. What occurs during stressful situations or cold temperatures?

A
  • 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
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16
Q

What occurs with iodine deficiency?

A
  • Hypothyroidism - insufficient dietary iodine to synthesize T4
  • Minor cause can be consumption of excessive goitrogens (4% cases) (anti-thyroid substances/foods); cabbage, spinach, radish, soybeans, peanuts, peaches, strawberries
  • Increased TSH = enlargement of thyroid follicles → goiter

Can’t turn off loop bc no T4 so body thinks we need more, keeps making thyroglobulin leading to excessive follicular cell expansion (happens with excessive plant substances)

17
Q

How do recent dietary trends impact iodine intake?

A
  • Increasing intake of plant-based foods and plant based protein (animal protein is higher in iodine)
  • Plant-based diets have the potential to be low in iodine (key vegetarian sources or iodine are iodized salt or sea vegetables) and diets containing less processed foods
  • Similar for plant-based alternatives (dairy products and milk have much higher iodine)
  • Vegetarian-based diets should consider supplementation or fortified alternatives
18
Q

How can iodine be used with radiation poisoning?

A
  • Uranium-235 decay can produce iodine-131 (very reactive, short half life)
  • Iodine is volatile - i.e. iodine-131 can be transmitted in the air
  • Iodine-131 can reside in the thyroid and cause cancer
  • Iodine supplementation can saturate the thyroid with non-radioactive iodide and reduce/minimize absorption of radioactive isoforms (slowly displaces radioactive iodine)
  • Generally iodine supplementation should take place 48 hours before or up to 8 hours after radiation exposure
  • Note: dietary iodine overdosing can cause significant side effects (when not bound and retained in thyroid)
19
Q

Name 8 other effective uses for iodine

A
  • Mostly antiseptic and topical uses
    1. Pink eye: eye drops containing iodine (povidone-iodine)
    2. Foot stores: topical povidone-iodine
    3. Anti-bacterial surgical preparation: povidone-iodine
    4. Benign breast disease (fibrocystic breast disease): molecular iodine, reduces fibrous breast tissue
    5. breast pain (mastalgia): 3000-6000mg of molecular iodine for 5 months
    6. Swelling (inflammation) and sores inside the mouth (oral mucositis)
    7. Lumps in the thyroid: taking iodine by mouth can improve lumps on the thyroid called thyroid nodules
    8. Leg sores caused by weak blood circulation (venous leg ulcer): reduces fibrosis
20
Q

Why is povidone-iodine useful?

A

Can disrupt bacterial membrane due to high concentration of salt. Acts as an antiseptic/antimicrobial