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Thyroid Gland Endocrinology
- Mainly T4 released
- T3 formed in peripheral tissues by 5'-deiodinases mainly in
liver and kidney and pituitary
- hormones released by TSH from anterior pituitary (stimulated by TRH from
hypothalamus)
- TSH causes raised c.AMP
- T3 and T4 cause negative feedback inhibition of TSH release
- in presence of decreased iodide, iodide uptake increases
- thyroid hormones mainly act through binding of T3 to nuclear recpetors
- binding of hormones causes increased production of Na+, K+-
ATPase (little in spleen and testes)
- hormones stimulate basal metabolic rate
- act as growth hormones by increasing protein synthesis
- enhance adrenaline effects on carbohydrate metabolism
- low doses increase glycogen synthesis, high doses promote
glycogenolysis
- cholesterol synthesis and degradation increased by thyroid hormones, but
as degradation increased more,
more hormone means less cholesterol , phospholipid and triglycerides
- Hypothyroidism
- deficiency of thyroid hormone (hypothyroidism, cretinism)
- retardation of mental development
- dwarfism, if early onset
- tiredness and lethargy
- some weight gain
- myxoedema as mucopolysaccharides accumulate
- delayed tendon reflexes
- mild hypothermia
- Hyperthyroidism
- excess hormone (hyperthyroidism)
- nervousness, irritability
- heat intolerance
- increased sweating
- weight loss
- eyelids may retract
- tacycardia during sleep may lead to atrial arrythmias and congestive
heart failure
14.3 THYROID GLAND and IODOTHYRONINES
| development of thyroid gland |
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| gross and microscopic structure of thyroid gland;
vasculature |
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| secretion of thyroid hormones; iodine economy of the
thyroid; action of TSH |
different deiodinases; interactions with autonomic
nervous system; euthyroid sick syndrome |
| plasma transport, long half lives of T4, T3 |
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| peripheral metabolism of T4 to T3 and rT3 by liver,
kidney; clearance of iodothyronines |
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| T3 as the metabolically active hormone; T3 receptors |
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| actions of T3 on basal metabolic rate (protein,
carbohydrate & lipid metabolism), development and growth |
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| catabolic versus anabolic effects; negative feedback of
T3, T4 on pituitary and hypothalamus |
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| control: via TSH, iodine |
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| dysfunction: effects of excess (thyrotoxicosis; IgGs
in Graves disease), deficiency (cretinism, myxoedema); iodine deficiency;
thyroid resistance |
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