Iodine

Question 1

Why is iodine unique and important?

Answer 1

• 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

Question 2

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

Answer 2

• 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

Question 3

What is the significance of iodine in salt?

Answer 3

• 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)

Question 4

Name food sources and RDA of iodine

Answer 4

• In micrograms!!
• Most comes from seafood

Question 5

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

Answer 5

• (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

Question 6

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

Answer 6

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

Question 7

What is the function of thyroglobulin?

Answer 7

• 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)

Question 8

What are MITs and DITs and what can they form?

Answer 8

• 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)

Question 9

What thyroid hormones are active or non active?

Answer 9

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

Question 10

What do microsomal enzymes do? Where do they function?

Answer 10

• 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.

Question 11

Explain the structure of the thyroid

Answer 11

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

Question 12

Explain the process of thyroid hormone synthesis

Answer 12

• 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

Question 13

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

Answer 13

• 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

Question 14

What is the function of T3?

Answer 14

• 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

Question 15

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

Answer 15

• 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

Question 16

What occurs with iodine deficiency?

Answer 16

• 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)

Question 17

How do recent dietary trends impact iodine intake?

Answer 17

• 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

Question 18

How can iodine be used with radiation poisoning?

Answer 18

• 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)

Question 19

Name 8 other effective uses for iodine

Answer 19

• 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

Question 20

Why is povidone-iodine useful?

Answer 20

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