Thyroid gland Flashcards
describe the structure and location of the thyroid glands.
- lies against and around front larynx and trachea below thyroid cartilage.
- 2 lobes joined by isthmus from 2nd to 3rd trachea ring; bow tie shaped.
DIFFERENT TO PARATHYROID.
what is the arrangement of thyroid tissue?
- follicular cells arranged in spheres called thyroid follicles makes thyroglobulin.
- colloid (stores thyroglobulin extracellularly surrounding follicle) contains deposits of thyroglobulin.
describe the composition of thyroid hormones. Briefly mention the function of thyroglobulin in its formation.
- 2 tyrosines linked together with iodine (at 3rd/ 4th position).
- only molecules in the body that contain iodine.
*The two hormones (T3/ T4) are made though the linkage of two molecules of iodinated tyrosine on the thyroglobulin scaffold. Thyroglobulin acts as scaffold/ substrate in its formation, undergoing iodination and coupling.
what is the importance of thyroid peroxidase?
- enzyme important in regulating 3 reactions involving iodine.
- oxidation of iodine to iodide which needs H2O2.
- addition of iodine to tyrosine on throglobulin.
- coupling in generating thyroid hormones.
what is the significance of iodine uptake to our body?
- dietary iodine reduced to iodide before absorption in small intestine.
- thyroid hormones and precursors only molecules with iodine in body.
- iodine taken up from blood via sodium-iodide symporter/ iodine trap.
why is T4 converted to T3?
- T3 biological activity is 4 times that of T4, so most converted in liver & kidneys.
- T3 & T4 both transported in blood bound to thyroxine-binding globulin.
describe the negative feedback regulation of the thyroid hormones.
- high levels of TSH negatively feedbacks onto hypothalmus to inhibit further TRH onto anterior pituatiry.
- T3 & T4 negatively feedbacks and inhibits both hypothalamus & TRH and also anterior pituitary & TSH release.
describe the structure and function of TSH.
- glycoprotein hormone composed of alpha and beta subunit.
- stimulates iodine uptake and oxidation, thyroglobulin synthesis and iodination, colloid pinocytosis to cell, proteolysis of thyroglobulin, cell metabolism and growth.
*via induced GaS (adenylyl cyclase PKA) and also GaQ (PLC, calcium) to stimulate thyroid hormone synthesis and release.
what are some general actions of the thyroid hormone?
- increase in basal metabolic rate and heat production : increase mitochondria, enzymes synthesis.
- stimulation of metabolic pathways : lipid, carb.
- sympathomimetric effects : via AD, NAD.
- CVS in response to AD, NAD increased CO, HR, peripheral vasodilation.
- nervous system : myelination and neurone development.
describe the structure of the thyroid receptors.
- nuclear receptors which function as hormone-activated transcription factors.
- acts by modulating gene expression.
- receptors bind DNA in absence of hormone leading to transcription repression.
- hormone binding causes conformational change to activate which relieves the repression.
what exactly is measured to check thyroid hormone levels?
- free T4 / T3 : meaning hormone not bound to plasma proteins!!!
what is thyroid scintigraphy?
- TC used for isotope scanning of thyroid with gamma camera.
- 1 day half life, radiation exposure low.
- uptake suggets problems.
what is thyroid scintigraphy?
- TC used for isotope scanning of thyroid with gamma camera.
- 1 day half life, radiation exposure low.
- large uptake suggests problems like graves.
what is the purpose of antithyroid drugs?
- used to treat overactive thyroid in hyperthyroidism like graves.
- blocks thyroid hormone formation.
eg : carbimazole prevents thyroid peroxidase from coupling and iodinating tyrosines on thyroglobulin.
TAKES 2-3 MONTHS TO TAKE EFFECT as iodinised stores in body.
