Thyroid Anatomy and Physiology Flashcards
At what vertebral level can the thyroid gland be found?
Level of C5
What is the basic functional unit of the thyroid?
Colloid surrounded by follicular cells, parafollicular cells are associated with the main unit
What are the stages of thyroid hormone synthesis?
Thyroglobulin synthesis Uptake and concentration of iodide Oxidation of iodide to iodine Iodination of thyroglobulin Formation of MIT and DIT Secretion
What are the stages of the synthesis and storage of T3 and T4?
Iodine taken up by follicle cells
Iodine attached to tyrosine residues on thyroglobulin to form MIT and DIT
Coupling of MIT and DIT (makes T3), and of two DITs (makes T4)
Stored in colloid thyroglobulin until required
What drugs inhibit the formation of MIT and DIT?
Carbimazole and propylthiouracil
What are some features of thyroxine (T4)?
90% of secreted thyroid hormone, converted to T3 by liver and kidneys
What are some features of tri-iodothyronine (T3)?
10% of secreted thyroid hormone, four times more potent than T4, major biologically active thyroid hormone
How are T3 and T4 transported?
Bound to serum proteins = thyroid binding globulin (TBG), thyroxine binding pre-albumin (TBPA), albumin
In what form can T3 and T4 enter cells?
Only unbound (free) T3 and T4 are available to cells
What are T3 and T4 nearly entirely bound by?
Plasma proteins
What are some features of the binding of T3?
Bound 10-20 times less avidly by TBG and not significantly by TTR = T3 has rapid onset and offset of action
What does metabolic state correlate to?
Correlates more closely with free than with the total concentration in the plasma
How does free hormone concentration affect concentration of total hormones?
Concentration doesn’t necessarily vary directly with that of the free hormones
What are the targets of thyroid hormones?
Virtually every tissue in the body, active throughout life to alter gene transcription and protein synthesis
How do thyroid hormones increase basal metabolic rate?
Increase number and size of mitochondria, oxygen use and rates of ATP hydrolysis, and synthesis of respiratory chain enzymes
How do thyroid hormones impact temperature regulation?
About 30% of temperature regulation is due to thyroid hormone thermogenesis
How do thyroid hormones influence carbohydrate metabolism?
Increases blood glucose due to glycogenolysis and gluconeogenesis, increases insulin-dependent glucose uptake into cells
How is lipid metabolism impacted by thyroid hormones?
Mobilise fats from adipose tissue, increase fatty acid oxidation
What effect do thyroid hormones have on protein metabolism?
Increase protein synthesis
What growth factors require thyroid hormones?
Growth hormone releasing hormone (GHRH)
Glucocorticoid-induced GHRH release
GH/somatomedins
How are thyroid hormones involved in development of foetal and neonatal brains?
Myelinogenesis and axonal growth
How can thyroid hormone levels impact normal CNS activity?
Hypothyroidism = slows intellectual functions Hyperthyroidism = nervousness, hyperkinesis, emotional lability
How do thyroid hormones affect the body’s response to adrenaline and noradrenaline?
Increase responsiveness = increase number of receptors
How do thyroid hormones impact the heart?
Increase CV responsiveness = increased force and rate of contraction
What is the function of de-iodinases?
Degrade thyroid hormones
What are the different de-iodinases?
Type I (D1) = found in liver and kidney Type II (D2) = found in heart, skeletal muscle, CNS, fat, thyroid, pituitary Type III (D3) = found in foetal tissue and placenta, and brain (except pituitary)
What can cause thyroid hormone deficiency?
Primary (gland) failure = may have goitre
Secondary to TRH and TSH = no goitre
Lack of dietary iodine = may have goitre
What are the symptoms of hypothyroidism?
Reduced BMR, slow pulse, fatigue, lethargy, slow response time and mental sluggishness, cold intolerance, easy weight gain
What causes Grave’s disease?
TSI act like TSH but go unchecked by T3 and T4 = autoimmune hyperthyroidism
What are some signs of Grave’s disease?
Exphthalmos = bulging eyes due to water retaining carbohydrate build up behind eyes
Goitre
What are the symptoms of hyperthyroidism?
Increased BMR, very fast pulse, increased nervousness and excessively emotional, insomnia, sweating and heat intolerance, lose weight easily
