Physiology of the Thyroid Gland Flashcards
Describe what colloid is in terms of tructure of the thyroid gland.
colloid = tyrosine containing thyroglobulin filled spheres enclosed by follicular cells
What do parafollicular C cells secrete and why is this important?
Secrete cacitonin - involved in cacium metabolism
Briefly describe how T3 and T4 are synthesised, stored and released.
Iodine moves into the follicular cell from the blood
This attaches to tyrosine residues on thyroglobulin to form T3 and T4. These can be secreted by pinocytosis or stored in colloid thyroglobuin until required
What exactly is T3 made up of?
MIT and DIT
If only one iodine joins the tyrosine residue what is created?
Monoiodotyrosine unit (MIT)
If 2 iodines join the tyrosine residue, what is the resulting molecule called?
Di-iodotyrosine unit (DIT)
If two DIT units join together, what is made?
T4
What is the method of action of Carbimazole and Propylthiouracil?
Prevent iodine attaching to tyrsine residues to stop the manufacturing of T3 and T4 - used to treat hyperthyroidism
Which thyroid hormone is secreted the most?
T4 = thyroxine - ~90% of thyroid hormones
Which thyroid hormone is most potent?
T3 (4x more potent than T4)
Can T4 be converted to T3?
yes in the liver and kidney
Which thyroid hormone is the major biologically active thyroid hormone?
T3
What is the main regulator of thyroid hormone release?
Thyroid stimulating hormone (TSH)
Are T3 and T4 hydrophilic or hydrophobic?
Hydrophobic and lipophilic so bind to plasma proteins
What is the main plasma protein that T3 and T4 bind to?
Thyroxine binding globulin (TBG)
What other plasma proteins can T3 and T4 bind to?
Thyroxine bindding prealbumin (~20%)
Albumin (~5%)
The bound/unbound thyroid hormone is biologically active adn therefore available to the tissue?
Unbound
What allows the release of T3 and T4 from the colloid?
Thyroid stimulating hormone (TSH)
Which thyroid hormone has more rapid onset and offset kinetics?
T3
Does the metabolic state correlate more closely with the free hormones or total hormone concentration?
Free
Do thyroid hormones increase or decrease basal metabolic rate?
Increase
Do thyroid hormones increase or decrease thermogenesis?
Increase
What is the effect of thyroid hormones on carbohydrate metabolism?
Increase blood glucose
Increase insulin-dependent glucose uptake into cells
What is the effect of thyroid hormones on lipid metabolism?
Mobilise fats from adipose tissue
Incerase fatty acid oxidation in tissues
What is the effect of thyroid hormones on protein metabolism?
increase protein synthesis
How do thyroid hormones indirectly effect growth?
growth hormone releasing hormone and growth hormone itself required thyroid hormones
In the development of foetal and neonatal brain, what exactly requires thyroid hormones?
Myelinogenesis and axonal growth require thyroid hormones
What is the effect of thryroid hormones on permissive sympathomimetic action?
Increase responsiveness to adrenaline and NA by increasing number of receptors
What class of drugs can be given to treat symptoms in initial stages for hyperthyroidism?
Beta-blockers (e.g. Propanolol)
How is the production of thyroid hormones regulated?
- Thyrotrophin releasing hormone (TRH), stimulates thyroid stimulating hormone (TSH) release from anterior pituitary
- TSH = major stimulant to release of T3 and T4 from thyroid gland
- T3 and T4 exert negative feedback control of release of TRH and TSH
What are the other 3 minor regulators of thyroid hormone release?
Low temperature
Stress
Circadian rhythm
What does the thyroid secrete?
Throxine (T4)
Tri-iodothyronine (T3)
Calcitonin
What do the parathyroid glands secrete and what does this act on?
parathyriod hormone (PTH) Calcium metabolism
The Hypothalamus secretes … that acts on the anterior pituitary to secrete…?
What does this allow the release of
TRH
TSH
T3&T4
What are the parathyroid cells composed of?
Chief cells supported by oxyphil cells
Describe the cell histology of a normal parathyroid gland.
Round cells with moderate cytoplasm and bland round central nuclei
