Flashcards in Lecture 21: Thyroid Hormones Deck (36):
How many mg of iodine (iodides such as sodium iodide) are required each year?
True or False:
Iodine is absorbed from the gut tract in the same manner as chlorides.
Is the sodium-Iodide symporter (2:1) located on the basal or apical membrane of the thyroid cell?
Where does the energy to drive the sodium-iodide symporter come from?
sodium-potassium ATPase pump
How much more concentrated can iodide be inside the cell compared to its concentration in the plasma?
What is the chloride-iodide counter-transporter also known as?
What transporter pumps iodide across the apical membrane of the thyroid cell into the follicle?
**aka - Pendrin**
What membrane bound enzyme catalyzes the conversion of iodide into iodine (I2)?
What enzyme catalyses the combing of iodide with thyrosine on thyroglobulin?
What does the combining of iodine with tyrosine on thyroglobulin result in?
- Monoiodotyrosine (MIT)
- Diiodotyrosine (DIT)
**catalyzed by peroxidase - which is a membrane enzyme**
What thyroid hormone is produced by the recombination of DIT + DIT?
**more T4 is formed than T3**
What thyroid hormone is produced by the recombination of DIT + MIT?
**more active than T4 - but more T4 is formed**
What is attached to a thyroglobulin molecule when it is stored in the colloid until needed?
Study and know figure 77-2 well for the exam.
What does thyroid cells secrete into the follicle?
How many tyrosine amino acids does each thyroglobulin molecule have that binds iodine to form thyroid hormones?
70 tyrosine amino acids
What form are iodide ions converted to so that they can directly bind to tyrosine amino acids on thyroglobulin molecules to form thyroid hormones
Iodide ions are converted to an oxidized form of iodine to directly bind to tyrosine.
> nascent iodine (I2) or
What is the major hormonal product formed during iodine metabolism to produce thyroid hormones?
How many molecules of thyroxine does each thyroglobulin molecule contain?
30 molecules of thyroxine.
True or False:
The thyroid can store several months supply of thyroid hormones.
Go through the mechanism of the release of thyroid hormones.
> Apical surface of thyroid cells pinch off small portions of the follicular colloid to form pinocytic vesicles.
> Lysosomes fuse with vesicles and digest thyroglobulin molecules to release thyroxine and triiodothyronine (thyroid hormones).
> Thyroid hormones diffuse into blood.
> Most of the iodinated tyrosine in the thyroglobulin does not become thyroid hormones but is recycled using a deiodinase enzyme (deficiency mimics iodine deficiency).
> Most of released thyroid hormone is thyroxine which then is slowly deiodinated to form triiodothyronine.
> Conversion from T4 to T3 requires 5'-iodinase
> T3 is more active than T4.
> Most thyroid hormones entering blood are 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 to tissue cells.
What enzyme is required for the conversion of T4 to T3?
What happens to most of the iodinated tyrosine in the thyroglobulin molecule?
Most of the iodinated tyrosine in the thyroglobulin does not become thyroid hormones but is recycled using a deiodinase enzyme.
**a deficiency of deiodinase enzyme mimics iodine deficiency**
What are most thyroid hormones that enter the blood stream immediately bound to?
Plasma proteins (thyroxine-binding globulin (TBG)) that are synthesized in the liver.
**plasma proteins have a high affinity for thyroid hormones, so the hormones are released slowly to tissue cells**
What signaling mechanism does TSH act on follicular cells via?
Adenylate cyclase - cAMP mechanism.
What is the regulation of thyroid hormone secretion starting in the hypothalamus?
> hypothalamus -> TRH (thyrotropin-releasing hormone)
> TRH -> anterior pituitary -> TSH
> TSH -> thyroid gland -> thyroxine
> thyroxine -> increased cellular metabolism
*increased cellular metabolism provides an inhibitory feedback to the hypothalamus and the anterior pituitary**
What are the causes of hyperthyroidism?
- 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.
What are the 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
What are the treatments for hyperthyroidism?
- Surgical removal of most of thyroid gland.
- Treatment with radioactive iodine.
What are the causes of hypothyroidism?
- Hashimoto disease (autoimmunity)
- Endemic goiter
- Cretinism (caused by extreme early hypothyroidism)
What are the symptoms of hypothyroidism?
- Generally opposite those of hyperthyroidism.
What are the treatments of hypothyroidism?
What do the antithyroid drugs thiocyanate and perchlorate anions do?
Inhibit iodine uptake (inhibit I-Na cotransport).
What does the antithyroid drug propylthiouracil do?
- inhibits peroxidase
- can be used to treat for hyperthyroidism
What does the antithyroid drug iodides do?
Decrease thyroid activity when present in very high concentrations.