T3 Thyroid Hormones Flashcards
How is iodine absorbed?
From the gut tract in the same manner as chlorides
Describe the iodide pump.
Sodium-iodide symporter (2:1) in basal membrane of thyroid cells.
Iodide is transported across apical membrane of cell into follicle via a chloride-iodide counter transporter. What is the transporter?
Pendrin
What catalyzes the conversion of iodide into iodine?
membrane enz peroxidase
The combination of iodine with tyrosine on thyroglobulin is catalyzed by what? What are the results?
Peroxidase; MIT and DIT
What makes up T4/thyroxine?
DIT + DIT
What makes up T3?
MIT + DIT
What do thyroid cells secrete into follicle?
thyroglobulin
Each thyroglobulin molecule has 70 tyrosine aa which bind _______ to form thyroid hormones.
iodine
Iodide ions are converted to a(n) [Oxidized or Reduced] form of iodine
oxidized
What is the major hormonal product of iodine metabolism?
thyroxine/T4
How thyroglobulin contains about ______ molecules of thyroxine.
30
The thyroid can store several months supply of what?
thyroid hormones
What is the first step in release of thyroid hormones?
apical surface of thyroid cells pinch off small portions of follicular colloid to form pinocytic vesicles.
In the second step of thyroid hormone release, the second step is that lysosomes fuse with vesicles and digest thyroglobulin molecules to release thyroxine and T3. What is the next step?
thyroid hormones diffuse into the blood.
Conversion from T4 to T3 requires what enzyme?
5’-iodinase
Which is more active T3 or T4?
T3
Most thyroid hormones (T3&T4) entering blood are immediately bound to plasma proteins synthesized where?
in the liver.
What is the protein formed by the liver that binds thyroid hormones? what is the effect if there is hepatic failure?
Thyroxine-binding globulin (TBG); hepatic failure = decreased total thyroid hormone level but normal free hormone levels
Plasma proteins have a _______ affinity for thyroid hormones, so the hormones are released slowly to tissue cells
high
What hormones have the following effects on the body?
- increased transcription of large number of genes
- increased cellular metabolic activity
- effect on growth
- stimulation of carbohydrate and fat metabolism
- effect on plasma and liver fats
- increased requirement for vitamins
- increased BMR
- decreased body weight
- along with SEVERAL others
thyroid hormones
What is the main effect of thyroid hormones?
increased basal metabolic rate
Thyroid hormones act on TSH, cAMP, and what others?
TRH, Cold and other neurogenic stimuli, and thyroid hormone feedback.
What brains structure acts on thyrotropin releasing hormone?
hypothalamus
Where does TRH act?
anterior pituitary
TSH is controlled by what brain structure?
anterior pituitary
TSH acts on the thyroid gland to produce what?
thyroxine
What is the effect of thyroxine?
increased cellular metabolism
Increased cellular metabolism has an inhibitory effect on what two things?
hypothalamus and anterior pituitary
Thyroxine has an inhibitory mechanism on which structure? Cerebellum Cerebrum Anterior pituitary Hypothalamus
Anterior pituitary
What is the mechanism of TSH acting on follicular cells?
via adenylate cyclase-cAMP
What are the causes of hyperthyroidism?
Graves’ Disease and Adenoma
What is Graves’ disease?
high concentration of circulating thyroid-stimulating immunoglobulins:
- IgG components
- mimic TSH and bind to TSH receptors
- result in high concentrations of circulating thyroxine and low levels of TSH
High state of excitability, intolerance to heat, increased sweating, mild to extreme weight loss, varying degrees of diarrhea, muscle weakness, extreme fatigue, inability to sleep, and tremor in hands are all characteristics of what condition?
hyperthyroidism
What are the treatment options for hyperthyroidism?
- surgical removal of most of the thyroid gland
- treatment with radioactive iodine
What are the causes of hypothyroidism?
Hashimotos, endemic goiter, cretinism
What are the symptoms of hypothyroidism?
- generally opposite those of hyperthyroidism
- myxedema
- atherosclerosis
What is the treatment of hypothyroidism?
oral medication
