Thyroid Gland Flashcards
What cells of the thyroid produce T3 and T4?
Follicular cells
What is thyrocalcitonin also known as?
Calcitonin
What cells of the thyroid produce calcitonin?
Parafollicular cells
Outline the synthesis of T3 and T4.
- Iodide trapping
- Synthesis of thyroglobulin
- Oxidation of iodide
- Iodination of thyroglobulin
- Coupling of T1 and T2 to form T3 and T4
- Digestion of colloid.
- Secretion of thyroid hormones into blood
- Transport of T3 and T4 in the blood by TBG
Explain iodide trapping.
Thyroid follicular cells trap iodide ions by tansporting them from the blood into their cytosol.
Where is thyroglobulin synthesised?
Follicular cells in the thyroid
Explain the iodination of thyroglobulin.
Iodine atoms bind to tyrosine on thyroglobulin producing T1 and T2.
Where is T4 converted to T3?
In the kidneys and liver
What is thyroglobulin known as once it has bound to one or two iodine atoms?
Colloid
Explain the digestion of colloid.
Digestive enzymes breakdown thyroglobulin cleaving off T3 and T4 .
How are T3 and T4 secreted?
They are secreted in response to TSH. They are lipid soluble molecules and so can diffuse through the plasma membrane into the blood.
How are T3 and T4 transported in the blood?
T3 and T4 are transported in the blood by the transport protein TBG (thyroxine binding globulin).
What is TBG?
Thyroxine binding globulin
What is TGB?
Thyroglobulin
How do the concentration of T3 and T4 in the blood differ?
Much more T4 is present in the body, however T3 is much more potent.
What are the three main actions of T3 and T4?
- Increased basal metabolic rate
- Enhanced action of catecholamines
- Regulation of the growth and development of nervous tissue and bones.
How do T3 and T4 increase basal metabolic rate, and what is the calornergic effect?
T3 and T4 increase the number of enzymes involved in cellular respiration, they also increase the number and activity of mitochondria in cells.
- –> this leads to increased ATP production, as more ATP is produced more heat is given off by the body.
- –> This is known as the calornergic effect. Thus the thyroid hormones have an important role in thermoregulation.
How do T3 and T4 enhance the effect of catecholamines? What symptoms does this lead to in hyperthyroidism?
T3 and T4 up-regulate the beta-adrenergic receptors epinephrine and norepinephrine bind to.
—> This is why increased heart rate and increased blood pressure are symptoms of hyperthyroidism.
How do thyroid hormones effect nervous tissue?
Thyroid hormones promote synapse formation, myelin production and growth of dendrites.
How do thyroid hormones affect bone growth?
Thyroid hormones promote:
- The formation of ossification centres in bones.
- The synthesis of bone proteins.
- Promote the secretion of GH and IGF’s.
What can deficiency of thyroid hormones in fetal development lead to?
Mental retardation and stunted bone growth.
Explain the feedback loop that controls thyroid hormone secretion.
- Low levels of T3 and T4 in the blood stimulate the release of TRH from the hypothalamus.
- TRH is carried by portal veins in the hypophyseal portal system to the anterior pituitary.
- TRH stimulate thyrotrophs to secrete TSH.
- TSH is released into the blood and stimulates follicular cells to produce T3 and T4.
- T3 and T4 are released into the blood by follicular cells.
- Elevated levels of T3 and T4 inhibit the secretion of TRH and TSH through negative feedback.
What conditions can increase the secretion of thyroid hormones?
Conditions that increase the demand of ATP, such as a cold environment, hypoglycemia, and pregnancy.
What does calcitonin do?
Decrease the level of calcium in the blood.
How does calcitonin decrease the levels of calcium in the blood?
Calcitonin inhibits the actions of osteoclasts that break down the bones extracellular matrix. Calcitonin enhances the uptake of calcium and phosphates into the bone extracellular matrix.
What miacalcin?
Miacalcin is a potent form of calcitonin used to treat osteoporosis.
