Physio 3 Flashcards
active forms of thyroid hormone
thyroxine (T4) and triiodothyronine (T3) are active forms. synthesized from AA tyrosine and require iodine
thyroid gland
bilobulated. regulates vertebrate growth, development, and metabolism. numerous follicles store TH in their lumens. the follicles are surrounded by cells that make and secrete TH
thyroglobulin (TG)
most of the iodide in the thyroid is associated with TG.
how does iodide get into and out of the thyroid gland?
specific transport protein (2Na/I symporter). uses inward Na gradient as driving force, doesnt use ATP directly
regulation of thyroid hormones
controlled by release of TSH, thyroid-stimulating hormone. undernegative feedback control by the thyroid hormones T3 and T4.
synth of thyroid hormones
- thyroglobulin molecules are produced in ER and go into lumen of follicle.
- Iodide enters the thyrocyte through Na/I cotransporters and exits into lumen through I/Cl antiporters
- I- is oxidized to iodine by thyroid peroxidase and joins with tyrosine residue of thyroglobulin
- This forms MIT, and two iodine moieties form DIT. this is known as organification. DIT and DIT are coupled through thyroid peroxidase to make T4. T3 is made by MIT + DIT.
main secretion of the thyroid gland
T4 hormone. the majority of T3 is derived by T4
T3 vs. T4
T3 is less biologically active than T4. T4 is the prohormone for T3. T4 is used to treat hypothyroidism due to longer half life and better stability
TH entry and transcriptional effects
unknown method of entry. TH receptors expressed in all tissues of body. TH regulates metabolism of carbs, proteins, and lipids. TH binds RXR which regulates genes containing TREs
TH regulation of growth and development
TH act synergistically with growth hormone to promote bone formation. THs promote ossification and fusion of bone plates and bone maturation. THs have effects on CNS and are important for CNS development during perinatal period. Can effect BMR, DNA, mRNA.
effect of thyroid hormone on BMR
BMR up in hyperthyroidism, and down in hypothyroidism. . Hypothyroidism tends to activated anabolic enzymes, while hyperthyroidism tends to activate catabolic enzymes.
TRH and TSH regulation of thyroid function
TRH stimulates TSH release by activating a GCPR linked to PLC. This makes IP3 and mobilizes Ca. TSH stimulates TH synth by activating GPCR which leads to increased cAMP. T3 and T4 act on target tissues and exert negative feedback at level of anterior pituitary and hypothalamus. Dopamine and somatostatin exert inhibitory effects on TSH release
detecting primary thyroid disease
level of TSH is checked. Usually elevated in hypothyroidism due to lack of negative feedback from T3 and T4, and vice versa in hyperthyroidism
