Thyroid Flashcards
What are T3 and T4 synthesized from and what is required in the diet for their synthesis?
Synthesized from the amino acid tyrosine on thyroglobulin
Requires iodide from the diet
What is the inactive form of thyroid hormone?
reverse T3
What helps iodide get in the cell and does it require ATP?
2Na/I- symporter–> not a pump so doesn’t require ATP
Where is the majority of organic iodide in the body?
residing in the thyroid gland in association with colloidal thyroglobulin
Where does the majority of T3 and reverse T3 come from?
T4
What is most circulating TH bound to?
Thyroid-binding globulin
also transthyretin and albumin
What TH is more active and which is considered a pro hormone?
T3 is more biologically active/potent
T4 is often referred to as a pro hormone
What metabolizes THs in tissues such as the liver and kidney?
5’ peripheral deiodinases
Which TH is used to treat hypothyroidism and why?
T4– longer half-life and greater stability compared to T3
What does cytoplasmic 5’/3’-monodeiodinase do?
Converts T4–> T3
What does free TH do when it gets into the cell?
bind to nuclear DNA at thyroid response elements in the promoter region of genes regulating transcription of T3 and T4 genes
What does injection of T3 and T4 do to the basal metabolic rate? What is the difference in the effects btwn T3 and T4?
Increases BMR–THs have a slow onset and long duration of action
T3 acts 4x as rapidly as T4–2-3days for T3 and several days for T4
What role does THs play in bone formation?
promote ossification and fusion of bone plates and bone maturation
What does TH deficiency in infants cause?
mental retardation (cretinism)–slowed only if treatment is initiated shortly after birth and growth retardation–slowed by treatment with thyroxine(T4)
What effects does T3 have on organs in the body?
- -Growth–bone formation and maturation
- -CNS- maturation of CNS
- -BMR- increase Na/K ATPase/O2 consumption/heat production/BMR
- -Metabolism- Increase glucose absorption/glycogenolysis/gluconeogenesis/lipolysis/protein synthesis and degradation(net catabolic)
- -Cardiovascular- increase CO
How does hyperthyroidism effect BMR? Hypothyroidism?
BMR increases in hyperthyroidism
BMR decreases in hypothyroidism
What do hypothyroid women have to lookout for when supplementing with thyroxine?
osteoporosis
What does hyperthyroidism increase expression of and what effect does this have? What is used to treat?
B-adrenergic receptors–leads to enhanced sensitivity to circulating epi and NE
B-adrenergic antagonists-propranolol used to treat symptoms
What increases in hypothyroidism and what does this cause?
increase in serum cholesterol which increases risk for atherosclerosis
How does TRH stimulate TSH release?
TRH activates G-protein coupled receptor linked to PLC–leading to IP3 generation and mobilization of intracellular Ca
How does TSH stimulate TH synthesis/release?
activating GPCR linked to adenylate cyclase–generating intracellular cAMP
What 2 substances inhibit TSH release?
Dopamine–on anterior pituitary and somatostatin– on hypothalamus
What do the levels of serum TSH look like in hypothyroidism? Hyperthyroidism?
Hypothyroidism- TSH is significantly elevated due to lack of negative feedback by low circulating T3 and T4
Hyperthyroidism- TSH is reduced due to excessive negative feedback by high circulating T3 and T4
