Thyroid Hormones And Iodine Flashcards
Where is thyroid gland located
-Located in the neck
-its 10-20g and contains spherical follicles filled with colloid
-colloid contains thyroglobulin containing t3 and t4 hormone
-c cells make calcitonin
What cells make calcitonin
C cells
What’s calcitonin
Amino acid peptide hormone
What’s colloid
Mixture in which one substance made up of insoluble practicals is suspended throughout another substance.It has a dispersed phases and a continuous phase
Regulation of thyroid hormone release
-hypothalamus releases TRH(thyroid releasing hormone)
-this causes the anterior pituitary gland to release thyroid stimulating hormone
-thyroid gland releases thyroid hormones such as t3 and t4
-t3 and t4 enter blood and circulate to target cells
-this causes an increased cellular metabolism,heart rate
Iodine
Iodine requirement: 100 – 250 μg / day
• Dietary iodine is trapped by the thyroid gland to synthesise thyroid hormones:
~ T4 : Thyroxine (4 x iodine)
~ T3 : Triiodothyronine (3 x iodine)
Where are t3 andt4 stored
In the colloid
Effect of thyroid stimulating hormone
-TSH binding causes T3 and T4 to re-enter thyroid follicle cells and be released into blood
-T4 & T3 circulate in blood, bind to thyroxine binding protein
-T4 & T3 diffuse into cells where T4 is converted to T3 (the active form)
Which thyroid hormone is inactive
T4
T3 & T4 Synthesis
- Iodine is transported into the colloid
- TSH promotes thyroglobulin
synthesis - Thyroglobulin is iodinated
- Thyroglobulin containing T3 & T4 stored for 3 months
- TG-hormone complex taken up by cells
- Fusion with lysosome
- Secretion of T3 & T4
Distribution of thyroid hormones
• Thyroid hormone levels in plasma are very stable and vary little:
~ they maintain their effects rather than ‘trigger’ effects
• T4 and T3 circulate bound to thyroxine binding protein
~ T4 & T3 diffuse into tissue cells where T4 is converted to T3 which is the more biologically active form
• T3 enters the nucleus and binds to high affinity T3 receptors on DNA to regulate mRNA and protein synthesis
~ TR-α1 receptors widely distributed throughout the body
~ TR-β1 receptors mainly in liver but also in kidney
~ TR-β2 receptors in hypothalamus and pituitary
Link between thyroid hormones and metabolism
• Cold, particularly in children, stimulates TRH (thyroid releasing hormone) release from hypothalamus paraventricular nucleus:
• TRH promotes thyroid hormone synthesis and release:
~ increases basal metabolic rate and heat production
~ heat is produced by muscle and brown adipose tissue
• Metabolic rate is the rate at which cells use oxygen to release energy (via ATP) by oxidation of food molecules:
~ energy release is about 20 k joules / litre of 02 used
• Glucose oxidation releases CO2, metabolic H20 and ATP:
~ C6H1206 + 602 → 6C02 + 6H20 + 36 ATP molecules
glycolysis + Citric acid cycle + oxidative phosphorylation
Thyroid actions in cells
- T3 enters nucleus
- T3 binding releases the co- repressor (CoR) from the thyroid hormone receptor (TR)
- Co-activator (CoA) assists T3 gene activation
- Gene expression is regulated by T3
Hyperthyroidism
• Increased HR •
• Increased Ventilation •
• Increased Vascularization •
• Decrease Body Fat •
• Increased Cardiac •
Decreased HR
Decreased Ventilation Decreased Vascularization Increased Body Fat Decreased Cardiac Size
Hypertrophy (Size increase)
• Decreased Thyroid Size (due to reduced stimulation)
• Reduced TSH (no stimulus to release more!)
Hypothyroidism
-Decreased HR
-Decreased Ventilation Decreased Vascularization Increased -Body Fat Decreased Cardiac Size
-Increased Thyroid Size (due to increased stimulation)
-Increased TSH (Thyroid gland being stimulated to release more!)
Effect of t3 and t4 on metabolic rate
-increased metabolic rate thus :
– Thermogenesis
– Increased basal metabolic rate
– Increased O2 consumption
– Increased Cardiac Output and Ventilation
Other actions of thyroid hormones
• Thyroid hormone is necessary for normal growth and development, particularly in babies and children
• Some effects of thyroid hormone result from its stimulation of growth hormone release
• Lack of T3 & T4 (often due to dietary iodine deficiency) causes:
~ irreversible brain damage in children
~ impaired brain function in adults (low mood / mood swings)
• All UK children are tested for thyroid function soon after birth
When is foetal thyroid hormone released
~ foetal thyroid hormone is released from week 11-12 of gestation to promote growth and development of the foetal skeleton and nervous system
Clinical Features of Congenital Iodine Deficiency Syndrome
• Caused by fetal and lifetime thyroid / iodine deficiency:
-hearing loss
-poor cochlea development dysarthria (poor speech) -Muscle rigidity
-Mental development & low IQ bradykinesia (slow movement)
-All other hypothyroidism effects
What is myxoedema
-post natal hypothyroidism
~ due to thyroid / iodine deficiency / iodine transporter deficiency
~ autoimmune Hashimoto’s thyroiditis
~ causes facial swelling, hair loss, dry cold skin, cold sensitivity, weight gain, loss of appetite, constipation, poor memory & lethargy
What is Graves’ disease
-hyperthyroidism
~ autoimmune disorder in which an antibody mimics TSH and stimulates T4 & T3 release
~ causes enlarged thyroid (diffuse goitre) with bulging eyes (exopthalmos), heat intolerance, sweating & anxiety
