Thyroid Flashcards
T3/T4 are synthesized by what?
Thyroid follicle (central lumen specifically)
What is the thyroid follicle?
Thyrocytes (follicular cells) with central lumen
Calcitonin is synthesized by what?
Parafollicular cells
T3/T4 consist of?
Thyroglobulin and iodide
What is “iodide trapping”?
Thyroid stimulating hormone promotes intake of iodide into follicular cells
Where is thyroglobulin produced?
Follicular cell
What does thyroid stimulating hormone also stimulate?
Reuptake of T3/T4 back into cell to be released from thyroid gland
What percent of T3/T4 binds to thyroxine binding globulin
70%
What percent of T3/T4 binds to other protein carriers?
29%
What percentage of circulating T3/T4 is free T3?
0.3%
What percentage of circulating T3/T4 is free T4?
0.03%
What is “circulating storage” of T3/T4?
T3/T4 that is bound to carrier proteins
(inactive)
What happens when there is too much free T3/T4?
Hypothalamus releases less TRH and the anterior pituitary gland releases less TSH
What happens when there is too little free T3/T4?
Hypothalamus releases more TRH and the anterior pituitary gland releases more TSH
Which thyroid hormone is directly used by body tissue?
T3
How is T4 used by body tissue?
Metabolised in to T3 first
What enzyme converts T4 into T3 in cells?
D2
What does D2 monitor?
That there is enough free T3 readily available in the cell
What enzyme converts T4 into T3 in the liver/kidneys?
D1
What does D1 monitor?
That there is enough available free T3 in the plasma
What enzyme deactivates T3/T4?
D3
When T4 is deactivated what is it converted into?
rT3
When T3 is deactivated what is it converted into?
T2
What does D3 monitor?
That there isn’t too much T3/T4 in the plasma
How do thyroid hormones affect metabolism?
Stimulate energy consumption and metabolism which increases body temperature
Thyroid hormones are ___________ with the sympathetic nervous system
synergistic
How do thyroid hormones affect growth and development?
Critical for CNS and skeletal development
Works w/ growth hormone and reproductive hormones
Insufficient thyroid hormones in newborns can cause?
Irreversible skeletal and neurological impairment
What are the target tissues of thyroid hormones?
Heart, liver, pancreas, CNS, bones, adipose, GI tract
How do thyroid hormones affect the heart?
Increases HR and cardiac output
T3 increases sensitivity to sympathetic system (epinephrine)
How do thyroid hormones affect the pulmonary system?
Stimulate the brainstem to increase ventilation
How do thyroid hormones affect adipose tissue?
Stimulate lipolysis
How do thyroid hormones affect the liver?
Stimulate glycogenolysis, gluconeogenesis, ketogenesis, synthesis of VLDL
How do thyroid hormones affect the pancreas?
Directly stimulate alpha and beta cells to produce glucagon and insulin
How do thyroid hormones affect the CNS?
Increase CNS neuron sensitivity/excitability
Stimulate neuron growth and development
What is the “critical window” for newborns in. regards to diagnosing thyroid hormone insufficiency?
2 weeks
Sooner the DX/TX the better the outcome
How do thyroid hormones affect the bones?
Promotes bone growth
Stimulates osteoblast and osteoclast activity (homeostasis)
Hypothyroidism is linked with increased risk of?
Osteoporosis
How do thyroid hormones affect the GI tract?
Increases GI secretions and motility which promotes glucose absorption
What is the Wolff-Chaikoff Effect?
High plasma iodide levels will decrease secretion of T3/T4
