Lecture 62-Thyroid Hormones 2 Flashcards
How does T3 provide negative feedback?
Suppressing TSH release, decreasing transcription of TSH, down regulating TRA receptors
Where is negative feedback predominatantly exerted? What is mostly decreased?
Anterior pituitary
Decrease TSH
What are two ways to inhibit TSH?
- Tonically inhibited by dopamine and somatostatin from hypothalamus
- Cortisol and GH
What happens to thyroid hormone secretion during fasting?
TASH response to TRH will diminish
T3 falls
Metabolic rate decreases
What happens to thyroid hormone secretion during meal or cold exposure?
Increases T3 availability
What does 5’ deiodinase convert?
What does 5 deiodinase convert?
5 prime converts T4 -> T3
5 converts T4 -> rT3
How does starvation effect conversion of T4 to T3?
5’ deiodinase in most tissues is inhibited, lower basal metabolic rate of O2 consumption
Brain 5’ deiodinase not affected
Where are major sites of degradation of thyroid hormones?
Liver, kidneys, skeletal muscle
Where are SOME T4 excreted?
In bile
How does T4 enter the cell to be deiodinated?
Enters cell via facilitated diffusion
Where does the T3-receptor complex bind to in order to stimulate transcription of genes?
Binds to thyroid-regulatory element on DNA
What effect do thyroid hormones have on cardiovascular effects?
Increases CO, PP and synthesis of adrenergic receptors
What effect do thyroid hormones have on thermogenesis?
Increase
What effect do thyroid hormones have on oxygen consumption?
Increase and increased Na/K ATPase
Increase in cell activity
What effect do thyroid hormones have on metabolism? (4)
- Increase supply of substrates for production of energy (increase gluconeogenesis, increase absorption of glucose from gut)
- Increase appetite, gut absorption and motility
- Increase mobilization of carbohydrates and fat (increase lipolysis)
- Protein synthesis (if thyroid hormone levels are physiological) and protein degradation (if thyroid hormone levels are high)
What effect do thyroid hormones have on growth?
Required for normal growth
What effect do thyroid hormones have on central nervous system?
-essential for normal maturation of CNS
-increases blood flow and glucose metabolism in brain
-enhances peripheral nerve reflexes
What effect do thyroid hormones have on autonomic nervous system ?
Increase cardiac output
Similar to sympathetic nervous system
What is hyperthyroidism?
Overactive thyroid , excess thyroxine
What causes hyperthyroidism in cats?
Adenoma, adenocarcinoma, thyroid disrupters in environment
What causes hyperthyroidism in horses and dogs?
Adenocarcinoma
If hyperthyroidism is in the thyroid gland what do the TSH levels look like? What if the disorder is in the hypothalamus or pituitary?
thyroid gland -> TSH is low (more common) due to negative feedback, high T3/T4 constantly suppressing TSH release
Hypothalamus/pituitary -> TSH is low, overproducing TSH
What is hypothyroidism defined as?
Underactive thyroid, low thyroid hormone
What is congenital cretinism? (Aka thyroid dwarf)
The lack of enzymes in thyroid gland to make hormone
If hypothyroidism is in the thyroid gland what do the TSH levels look like? What if the disorder is in the hypothalamus or pituitary?
Thyroid gland -> TSH is high
Hypothalamus/pituitary gland -> TSH low
What is measured in blood tests to check the thyroid?
TSH, and T4 (T3 is typically NOT measured)
