Thyroid Physiology Flashcards
Which hormones does the thyroid gland secrete?
Triiodothronine (T3)
L-thyroxine (T4)
Calcitonin
Which hormone does the parathyroid gland secrete?
Parathyroid hormone
What is the name of the basic functional unit of the thyroid gland?
Follicle
Which cells produce calcitonin?
Parafollicular cells
How does the majority of T3 and T4 exist in the blood?
It is bound to hormone-binding proteins.
What is the function of free thyroid hormones?
It is available for action at target tissues
What is the negative feedback system in place in the thyroid?
- The HYPOTHALAMUS secretes TRH.
- This causes the PITUITARY to secrete TSH.
- This causes the THYROID to secrete T4 and T3.
- T3 and T4 have a negative feedback mechanism
What happens to T4 in the peripheral tissues?
It is converted to T3
What are the 6 steps to thyroid hormone synthesis?
- Thyroglobulin synthesis
- Uptake and concentration of Iodine I-
- Oxidation of I- to Iodine
- Iodination of thyroglobulin
- Formation of MIT and DIT
- Secretions
Where is T3 and T4 stored?
Colloid thyroglobulin
What is combined to form T3?
One monoiodotyrosine and one diiodotyrosine
MIT + DIT
What is combined to form T4?
2 Diiodotyrosine
2 DITs
Which thyroid hormone is the most potent?
T3 is 4 times more potent than T4
What is the major biologically active thyroid hormone?
T3
What should we measure in order to determine thyroid function?
free T3 and T4
Which states increase the number of thyroid binding globulins?
Pregnancy New Born Hepatitis A Biliary Cirrhosis Genetically Determined
Which states decrease the number of thyroid binding globulins?
Androgens
Severe systemic illness
Chronic liver disease
Genetically determined
What are the effects of the thyroid hormones?
They have the overall effect of increasing the basal metabolic rate. Increase net glucose release Increase lipolysis Decrease fat formation Increase heart rate Increase heart contractility Increase breathing Increase protein synthesis
How do the thyroid hormones increase the BMR within the cells?
Increase the number and size of mitochondria
Increase O2 use and rates of ATP hydrolysis
Increase synthesis of respiratory chain enzymes
What are the effects of thyroid hormone on growth and development?
Growth hormone releasing hormone production and secretion requires thyroid hormones
What is the sympathetic action of thyroid hormones?
They increase responsiveness to adrenaline and noradrenaline by increasing the numbers of receptors
Why can beta blockers be used to treat the initial symptoms of hyperthyroidism?
Because of its nervous action
Where are the three types of de-iodinases located?
Type 1 - commonly found in the liver and kidney
Type 2 - found in the heart, skeletal muscle, CNS, fat, thyroid, and pituitary
Type 3 - found in foetal tissue, placenta and brain
How are thyroid hormones degraded?
Inactivation of thyroid hormones occurs by removal of an iodine atom on the inner ring, which converts thyroxine to the inactive reverse triiodothyronine (rT3), or which converts the active triiodothyronine to diiodothyronine (T2).
What is the difference between thyroid disease and thyroid syndrome?
In thyroid syndrome there is excess cortisol in secretion but no underlying pituitary abnormalities
