Exam 5: Thyroid Physiology Flashcards
What is required for T3/T4 synthesis?
Iodide
What is iodide absorbed from?
GI tract
What is iodide excreted by?
Kidney
Where does iodide uptake occur?
Thyroid gland
Salivary gland
Describe the iodide pump of the thyroid gland
Active transport (energy dependent) Concentration gradient up to 250x
What is the uptake of iodide by the thyroid gland dependent on?
TSH
What does thyroid peroxidase play a role in?
Iodide oxidation
Organification
Coupling
Which is more active, T3 or T4?
T3
What makes T3 more active?
It enters the cell more rapidly and it is more potent
How much T3 is made in the thyroid? Where does the rest come from?
20%
De-iodination of T4 in tissues
What is necessary for T3 and T4 transport?
Protein-binding
What percentage of T3 and T4 are protein bound?
99%
What determines that half life of thyroid hormone?
Binding affinity
Which type of hormone is active, bound or free?
Free
What is the biologically active version of thyroid hormone?
Free T4
Free T3
How does thyroid hormone effect metabolism?
Increases metabolic rate, which increases oxidative phosphorylation in mitochondria
What are the catabolic effects of thyroid hormone?
Lipolysis
Protein
Carbohydrate utilization, GI absorption
What does thyroid hormone do to gastrointestinal motility?
Increases it
How does thyroid hormone effect the cardiovascular system?
Stimulation of cardiac β-adrenergic receptors (increase HR, CO)
Increase GFR
Stimulates erythropoiesis
How does thyroid hormone effect the neuromuscular system?
Increase mental alertness
Necessary for normal growth and neuronal development
What does the hypothalamus receive input from?
Higher centers in the brain and body
What hormone comes from the hypothalamus in the HPT axis?
Thyroid releasing hormone (TRH)
What does TRH do?
Increase thyroid stimulating hormone (TSH)
What hormone comes from the pituitary gland in the HPT axis?
TSH
What does TSH do?
Binds to TSH receptors in thyroid, which stimulates the synthesis of T3 and T4
What hormones come from the thyroid in the HPT axis?
Thyroid hormones (T3, T4)
Describe thyroid hormone
Free hormone only
Inhibits TRH and TSH secretion
Increased T4 decreases TRH and TSH
