thyroid Flashcards
what is T3?
tri-iodothyronine
what is T4?
thyroxine
what does T3 and t4 do?
work together to regulate how body uses energy
which is active in T3 and T4?
- T4 is a reserve and converted to active T3
how do thyroid hormones travel in body?
hyroid hormones are bound to proteins in circulation eg thyroxine binding globulin (TBG)
what are normal levels of T4?
T4 – 100nmol/L total, free = 20pmol/L
what are normal levels of T3?
T3 – total – 2nmol/L and free = 5pmol/L
what does the thyroid do?
- Produces and stores thyroid hormones and sequesters iodine from blood
which cells produce thyroid hormones?
follicular cells
what do c cells produce?
- C cells (parafollicular) produce calcitonin
can shape and size of thyroid cells change?
- Size of follicles changes with the amount of stored material
how is iodine taken up into follicular cells?
- Iodine is taken into follicular cells by a TSH sensitive pump
what happens to iodine when taken into follicular cells?
- Iodine diffuses to follicle cavity, where thyroglobulin (containing T3 and T4) are exocytosis and then endocytosed
- Lysosomal digestion breaks down the thyroglobulin molecule and T3 and T4 are endocytosed out the cell
how are T3 and T4 majority in the blood stream?
- T3 and T4 bind to TBG, transthyretin or albumin and are in blood stream
what detects disturbance T3/4 homeostasis?
- Disturbances homeostasis are detected by hypothalamus – this releases TRH
- TSH is the secreted which is secreted to thyroid gland
TSH stimulates TH synthesis – requires tyrosine and iodine
how do is iodine intake ensured?
- Iodination of salt of 150ug/day – salt now contains iodine in developing countries to prevent this problem
what does thyroid hormone do to CVS?
- Increase HR, SV, CO and contractility of myocardium
- Moderate vasodilator to blood and tissues
what does thyroid hormone do to cell turnover?
- Promote cell turnover and metabolic activity in reproductive system – fertility
how is thyroid essential in development and growth?
- Essential for nomrla brain development in utero and growth in children
- Perceived energy levels and higher cognition
- Stimulate mitochondrial replication and activity
- Increase cellular demand for glucose
what does calcitonin do?
- Calcitonin acts to reduce extracellular calcium in blood
- Inhibits osteoclast activity
- Stimulates renal excretion of calcium
what does corticosteroids interact with?
inhibit TSH production/ release. Inhibit TBG production
how does dopaminergic drugs affect thyroid?
inhibit TSH production/ release
how does lithium affect thyroid?
inhibit T3/T4 production
how does carbimazole affect thyroid?
inhibit T3/ T4 production
how does oestrogen affect thyroid?
increase TBG (thyroxine binding globulin) and TSH
what is TBG?
thyroxine binding globulin
how does androgens affect thyroid hormones?
: inhibit T3 and T4 production
how does beta blockers affect thyroid?
decrease T4 to T3 conversion – not a primary method of management
what thyroid levels are seen in hyperthyroidism?
- High T4
- High T3
- Low TSH
what thyroid levels are seen in hypothyroidism?
- Low T4
- Low T3
- High TRH
