Iodine metabolism and thyroid hormones Flashcards
Describe the abundance of iodine?
High abundance in sea
Low abundance on land
What is the RDI for iodine?
150 ug
What is iodine required for in the body?
Synthesis of thyroid hormones T3 and T4
What can iodine deficiency lead to?
Decreased thyroid hormone synthesis > feedback to increase production of TSH > induces thyroid cell proliferation > goitre
What is the cause of cretinism?
Congenital condition due to maternal iodine defciciency
Which radioactive substance can destroy the thyroid?
Radiation from I-131
Why are iodine deficiencies common in humans and land animals?
Absence from soils
Describe the difference between an antiseptic and a disinfectant?
Antispetic: antimicrobial that reduces the risk of infection, applied externally to living surfaces
Disinfectant: antimicrobial that is applied to non-living objects
Can iodine be used as an antibiotic, antiseptic, or disinfectant?
Antiseptic
What are the advantages of using iodine as an antiseptic?
Wide spectrum of antimicrobial activity
When brown - can see where it is and inidicates that it is active
Describe why iodine has an antimicrobial action?
Active form, I2, is a potent oxidiser
Reacts in electrophilic reactions with enzymes of respiratory chain and amino acids in cell membrane and cell wall > cell integrity destroyed
Why is iodine used as an X-ray contrast material?
High atomic number
Low toxicity
Ease of attachment to organic compounds
Is iodine a major mineral or a trace mineral in the body?
Trace mineral (20mg/body)
Which substance has the highest concentration of iodine?
Kelp
What is the recommended upper level of iodine intake per day?
1100 ug/day
What are the most significant sources of iodine in the human diet?
Iodised salt
Seafood
Bread
Briefly describe the symptoms of iodine deficiency?
Underactive thyroid gland
Goitre
Cretinism
Describe the symptoms associated with iodine toxicity?
Underactive thyroid gland
Elevated TSH
Goitre
What are goitrogens?
Where are they found?
Substances that inhibit iodide uptake in the thyroid and magnify the severity of any iodine deficiency
Found in soy, cabbage, kale, brussel sprouts
How many people in the world are affected by iodine deficiency?
2 billion
How many people are affected by goitre?
200 million
How many people worldwide are affected by cretinism?
6 million
Most common form of mental retardation
Also most preventable
What is the most common and also most preventable form of mental retardation?
Cretinism
Describe the hypothalamic-pituitary-thyroid axis?
What controls the release of thyroid hormones?
TSH from pituitary
Describe the action of thyroid hormones?
Increase mental alertness
Increase basal metabolic rate
Elevate number of catecholamine receptors > enhance catecholamine effects
Stimulate differentiation and function of brown adipose tissue > generate heat
Why does every cell in the body rely upon thyroid hormones?
Required for regulation of metabolism
Describe the proportion in which the normal thyroid produces T3 and T4?
80% T4
20% T3
Describe the relative activity of T3 and T4 in the body?
T3 has about four times the activity of T4
Describe the half-lives of T3 and T4?
T3: 6-9 days
T4: 24-36 hours
Describe the synthesis of T3 and T4?
Iodide oxidised by thyroperoxidase > iodine reacts with tyrsosines in thyroglobulin > iodinated tyrosines on surface of thyroglobulin enzymatically condensed by thyroperoxidase > modified thyroglobulin taken up into thyroid cell > peptide links hydrolysed > free amino acids and T3/T4 liberated
Where is thyroperoxidase located?
Embedded in thyroid cell membrane
Describe the process of T3 and T4 secretion from the thyroid gland?
Describe uptake of iodide by the thyroid gland?
TSH stimulates TSH receptor > cAMP activated > activates NIS (Na iodide symport) > Na and I enter cell at same time
What drives the NIS in thyroid cells?
Na gradient maintained by 3Na/2K ATPase
Describe the use of radioactive iodine as treatment for goitre?
Iodine-131 can be used to reduce thyroid mass
Describe the treatment options for goitre?
Radioactive iodine treatment
Antithyroid drug therapy
Surgery/thyroidectomy
Thyroid arterial embolization
How are thyroid hormones transported in the plasma?
Why?
70-80% carried in plasma bound to thyroid binding globulin (TBG)
Remainder divided between transthyretin and serum albumin
Because it is lipophilic
Describe how thyroid hormones elicit their actions?
Receptors bind DNA > transcritptional repression in absence of hormone
Hormone binds receptor > transcription activated
Compare the effects of hypothyroidism and hyperthyroidism?
