Thyroid Hormones Flashcards
Describe the iodide pump.
- sodium-iodide symporter (2:1) in basal membrane of thyroid cell
- energy for pump comes from a sodium-potassium ATPase pump
- iodide can be concentrated inside the cell 30-250x plasma conc
- iodide is transported across apical membrane of cell via follicle via chloride-iodide counter-transporter -> PENDRIN
- membrane enzyme, peroxidase, catalyzes conversion of iodide into iodine
What two steps are required for the synthesis of thyroid hormones?
- organification of iodine
- recombination
How is iodine organified?
- combining of iodine with tyrosine of thyroglobulin
- catalyzed by peroxidase
- results in monoiodotyrsosine (MIT) and diiodotyrosine (DIT)
What happens during the recombination step?
- DIT + DIT = thyroxine/T4 (more T4 than T3)
- DIT + MIT = triiodothyronine/T3 (more active)
What is in thyroglobulin?
- has MIT, DIT, T3, and T4 attached to it and is stored in the colloid until needed
- has about 70 tyrosine aas which bind iodine to form thyroid hormones
How many molecules of thyroxine does thyroglubulin contain?
30
How much throws hormone does the thyroid contain?
-several months supply
How are thyroid hormones released?
- apical surface of. Thyroid cells pinch off small portions of the follicular colloid from pinocytotic vesicles
- lysosomes fuse with vesicles and digest thyroglobulin molcules to release thyroxine and triiodothyronine
- thyroid hormones diffuse into blood
Most of the iodinated tyrosine in the thyroglobulin does not become thyroid hormones but is recycled using a _________ enzyme.
Deiodinase (deficiency mimics iodine deficiency)
Most of the released thyroid hormone is ________ which is then slowly decoding tend to form ________.
- thyroxine
- triiodothyronine
What is required to convert T4 to T3?
-5’ iodinase
Which thyroid hormone is more active?
T3
What happens to thyroid hormones when they enter the blood?
-immediately bound to plasma proteins synthesized in the liver
+thyroxine binding globulin (TBG) -> produced by liver
+hepatic failure: decreased total thyroid hormone level but normal free hormone levels
-plasma proteins have a high affinity for thyroid hormones, so the hormones are released slowly into tissue cells
What do thyroid hormones do physiologically?
- increased transcription of a large number of genes
- increased cellular metabolic activity
- effect on growth
- effects on specific bodily functions
- effects on cardiovascular system
- increased GI motility
- excitatory effects on CNS
- effects on muscle function
- muscle tremor
- effect on sleep
- effect on other endocrine glands
- effect on sexual function
What do thyroid hormones do for bodily functions?
- stimulation of carb metabolism
- stimulation of fat metabolism
- effect on plasma and liver fats
- increased requirement for vitamins
- increased BMR
- decreased body weight
How do thyroid hormones effect the cardiovascular system?
- increased blood flow and cardiac output
- increased HR
- normal arterial pressure
- increased respiration
What is the pathway from the hypothalamus to thyroxine?
Hypothalamus -> TRH -> anterior pituitary -> TSH -> thyroid gland -> thyroxine -> increased cellular metabolism
What does increased cellular metabolism have negative feedback on?
- hypothalamus
- anterior pituitary
What does thyroxine have negative feedback on?
-anterior pituitary
What causes hyperthyroidism?
- Graves’ disease (high conc of circulating thyroid stimulating immunoglobins -> low levels of TSH)
- adenoidal
What are some symptoms of hyperthyroidism?
- high state of excitability
- intolerance to heat
- increased sweating
- mild to extreme weight loss
- varying degrees of diarrhea
- muscle weakness
- nervousness or other psychic disorders
- extreme fatigue
- inability to sleep
- tremor in hands
- exophthalmos
What is the treatment for hyperthyroidism?
- surgical removal of most of thyroid gland
- treatment with radioactive iodine
What causes hypothyroidism?
- Hishimoto disease (autoimmunity)
- endemic goiter
- cretinism (caused by early hypothyroidism)
What are the symptoms of hypothyroidism?
- opposite of hyperthyroidism
- myxedema
- atherosclerosis
What is the treatment for hypothyroidism?
-oral medication
What do thiocyanate and perchlorate anions do?
-inhibit iodine uptake (inhibit I-Na cotransport)
What does propylthiouracil do?
- inhibits peroxidase
- can be used to treat hyperthyroidism
What do iodide so do?
-decrease thyroid activity when present in very high conc said
How much iodine is required each year? How is it absorbed?
- 50mg
- absorbed from gut tract in same manner as chloride
