Thyroid Hormones Flashcards
thyroid hormones are comprised of:
colloid and follicular cells
colloid (protein) solution
contains thyroglobulin and stored thyroid hormone
Synthesis of T3 and T4 by thyroid gland
requires intake of iodine
- enters thyroid gland follicular cells via iodide trapping by sodium-iodide symporter (NIS)
- goes through pendrin protein than transport iodide in colloid
- thyroglobulin combines with iodide and attaches to TG
- iodide is coupled to make T3 and T4
fxn of thyroperoxidase
- catalyzes oxidation of iodide
- iodination of tyrosine
- coupling of iodotyrosines
secretion of thyroid hormones
- TSH stimulates synthesis and secretion
- enzymes hydrolyze bonds between thyroid hormones and thyroglobulin
- T3 and T4 enter circulation
- iodide recycled
regulation on thyroid hormone secretion
temp. has effect on TRH and thus thyroid hormone
stress decreases thyroid hormone
thyroid hormone transport
- thyroxine-binding globulin
- thyroid hormone half lives
- T3: 1 day
- T4: 6 days
T3 production
at tissue level
by de-iodination of T4 by type 1 and type 2 deiodinases
Type 1 deiodinase produces
kidney, liver
T3 by:
removing outer iodide
activity can be altered by burns, tramua, cancer, etc
Type 2 deiodinase produces
muscle, brain
T3 by:
removing outer iodide
Type 3 deiodinase produces
reverse T3 by:
removing inner iodide
at tissue level, T4 can be deiodinated by type 3 deiodinase to produce
reverse T3
inactive hormone
Intracellular effects of TH
- increased basal metabolic rate
- increased synthesis and activity of NaKATPase
- increased synthesis of metabolic enzymes: heat production, O2 consumption, ATP generation
systemic effects of TH
- mobilization of substrates for ATP generation by cells
- increased delivery of substrates to cells
- increased delivery of O2 to cells; CO2 removal from cells
thyroid hormone increases availability of substrates for cells
- exogenous substrates increase food intake
- endogenous substrates increase mobilization
- increased delivery of substrates to cells: increased CO
- increased respiratory rate
TH is permissive for full physiological effects of:
EPI, insulin, GH
hyperthyroidism
- weight loss
- tachycardia
- sweating
- diarrhea
- graves’ disease
- tumor of follicular cells
- abnormal TRH and/or TSH secretion
treatment of hyperthyroidism
decrease synthesis of TH
- inhibit iodide trap
- inhibit activity of thyroperoxidase
- surgical removal
- radioactive iodine to ablate tissue
Hypothyrodism
weight gain
bradycardia
cold intolerance
constipation
autoimmune destruction of thyroid gland: hashimoto disease
surgical removal of gland
hypothalamic or pituitary failure
diet low in iodine
