Thyroid and parathyroid gland Flashcards
Where is the thyroid gland located?
below the layrnx and anterior (front) to the trachea
name the 2 parathyroid glands
superior parathyroid gland
inferior parathyroid gland
What is the thyroid gland made up of?
colloids
What type of cells line colloid cells?
follicular cells
What do follicular cells do?
trap iodine
What are between follicular cells of colloids?
parafollicular cells (C cells)
What do parafollicular cells secrete?
calcitonin (peptide hormone)
What does calcitonin do?
decreases plasma calcium concentrations (has parathyroid hormone effects)
What connects the 2 loops of the thyroid gland?
the isthmus
What is the thyroid gland responsible for?
regulation of thryoid hormone secretion
What is the synthesis and regulation of thryoid hormones regualted by?
TSH and TRH
TRH
thyrotropin releasing hormone
What 2 hormones does the thyroid gland secrete?
T4 and T3
% of T4 and T3 that are secreted
90% T4
10% T3
What does T4 stand for?
thyroxine
What does T3 stand for?
tri iodo thyronine
T3 and T4 in circulation
bound to thyroxine-binding globulin (TBG), thyroxine-binding prealbumin or albumin
Are bound thyroid hormones active or inactive?
inactive
What do thyroid hormones affect?
growth and maturation of tissues
cell metabolism
heat production
oxygen consumption
Where are the parathyroid glands located?
behind the upper and lower poles of the thyroid gland
What do the parathyroid glands produce?
parathyrid hormone
What does parathyroid hormone do?
regulator of serum calcium
antagonist of calcitonin
What is calcium important for?
conduction of electrical impulses in nervous and muscular systems
What is the only element/mineral that has its own regulatory system?
calcium
What regulates calcium in the body?
parathyroid gland
What happens when there is a decrease in serum calcium?
increased PTH (parathyroid hormone) secretion
What does PTH do in the body?
acts on bone, kidney, gut to icnrease serum calcium levels
normal range of plasma calcium
9-10 mg/dl
normal calcium levels in the body in mmol/L
2.25 mmol/L
What can alterations of parathyroid function cause?
hyperparathyroidism and hypoparathyroidism
What is primary hyperparathyroidism?
excess secretion of PTH from one or more parathyroid glands
What happens if there is an imbalance in calcium homeostasis of the blood?
high Ca levels:
- thyroid gland releses calcitonin
- calcitonin stimulates calcium salt to deposit in bone
- Ca levels fall
low Ca levels:
- parathyroid glands release parathyroid hormone (PTH)
- osteoclasts degrade bone matrix and release Ca into blood
- raises Ca levels
What is secondary hyperparathyroidism?
increase in PTH secondary to a chronic disease
What is hypoparathyroidism and what causes it?
abnormaly low PTH levels
usually caused by parathyroid damage in thyroid surgery
What usually causes hyperparathyroidism?
due to a benign growth of 1:4 parathyroid glands
What does hyperparathyroisidm cause?
induces abnormally high cerum Ca levels
bone decalcification
development of kidney stones
signs of hyperparathyroidism?
fatigue apathy (lack of interest/enthusaism) muscle weakness vomiting hypertension demineralisation of bones development of kidney stones
When does hypercalcemic crisis occur?
when Ca levels are over 15 mg/dl (3.75 mmol/L)
What sympoms does hypercalcemic crisis cause?
neurological
cardiovascular
renal symptoms
What treatment is used for hypercalcemic crisis?
treatment to decrease serum calcium
- IV fluids (saline 0.9% 2-4L/day)
- phosphate therpay (IV)
- calcitonin (inc excretion of Ca)
- dialysis (if the others don’t work after 48hrs)
What causes hypothyroidism?
accidental removal of parathyroid glands during thyroid surgery
What are hypoparathyroidism symptoms casued by?
hypocalcemia and hyperphosphatemia
Symptoms of hypoparathyroidism
neuromuscular irritability
tetany
- numbness, tingling, cramps
- bronchospasm, laryngeal spasm, carpopedal spasm
What is carpopedal spasm?
involuntary contraction of the feet/hands
management of hypoparathyroidism
calcium gluconate
10-20ml 10% IV over 10-20 mins
What transporters take iodide from the blood into the lumen? (2)
- sodium iodide symporter (NIS)
2. pendrin 1 (PDS) (iodide chloride transporter)
What is the oxygenation of iodide and its incorporatin into the thyroid globulin called?
organification of iodide
What enzyme catalyses the oxidation of iodide?
thyroperoxidase
What enzyme catalyses the oxidation of iodide?
thyroperoxidase
What enzyme catalyses the oxidation of iodide?
thyroperoxidase
What do tyrosine residues form in the thyroid globulin?
MIT - monoidothyrosine (position 3 on the ring)
DIT - diidothyroisine (at position 5)
3 main steps in thyroid hormone synthesis
- uptake of thyroid hormone into cells
- oxidation of iodide, iodination/organification and coupling
- endocytosis and seretion of thyroid hormones
3 main steps in thyroid hormone synthesis
- uptake of thyroid hormone into cells
- oxidation of iodide - iodination and coupling
- endocytosis and seretion of thyroid hormones (T3 &4)
3 main steps in thyroid hormone synthesis
- uptake of thyroid hormone into cells
- oxidation of iodide - iodination and coupling
- endocytosis and seretion of thyroid hormones (T3 &4)
3 main steps in thyroid hormone synthesis
- uptake of thyroid hormone into cells
- oxidation of iodide - iodination and coupling
- endocytosis and seretion of thyroid hormones (T3 &4)
3 major thryoid-binding proteins
TBG - thyroid binding globulin
TBPA - thyroxine binding prealbulin
albumin
What 2 molecules form to make T3?
MIT and DIT
What molecules form to make T4?
2 DIT molecules
How much T4 and T3 does TBG (thyroid hormone-binding globulin) carry?
70% of T4 and T3
How much T3 and T4 does TBPA carry?
10% of circulating T4 and less amounts of T3
How much T3 and T4 does albumin carry?
15% of circulating T3 and T4
Is T3 or T4 more active?
T3
How is T4 converted to T3?
an iodine is removed from the outer ring of T4 to give T3
What removes and iodine from T4 to give T3?
5/3 deiodinase
What is rT3?
reverse T3
-> inactive
What does 5’/3’ deiodinases do?
removes an iodine from the inner ring of T4 to give reverse T3 (rT3)
Where is TRH synthesised and released from?
hypothalamus
What secretes TSH?
anterior pituitary
What stimulates the secretion of TSH from the anterior pituitary?
hypothalamic TRH
What causes inhibition of TRH?
when there’s high levels of circulating thyroid hormone (T3 and T4)
What substances suppress TSH secretion?
dopamine
somatostatin
glucocorticoids
What are T3 and T4 bound to in the plasma?
thyroxine-binding globulin
How do T3 and T4 dissociate from thyroxine-binding plasma proteins?
diffusion or active transport
What happens to T4 when it enters the cell?
it is enzymatically deiodinated to T3 which enters the nucleus and attaches to specific receptors
What happens when T3 attaches to specific receptors in the nucleus?
the receptors are activated and it promotes the formation of RNA and protein synthesis
this is responsible for the effects of T4
functions of thryoid hormones
- increases metabolism and protein synthesis
- influences growth and development in children (mental development and sexual maturity)
How is thyroid hormone secretion regulated?
hypothalamus releases TRH
causes the anterior pituitary to release TSH
causes the thyroid gland to release T3 and T4
- this increases metabolism rate/heat production
- enhances growth/CNS development
- enhances sympathetic activity
