Quiz 12 Flashcards
____ temperatures will stimulate thyroid hormone production to increase
Cold
Describe the hypothalamic pituitary thyroid negative feedback loop. What is stimulatory what is inhibitory?
Hypothalamus secretes TRH which stimulates the anterior pituitary to secrete TSH which then stimulates the thyroid gland to produce thyroid hormones, which then go to the target tissues. TSH will inhibit the hypothalamus from producing TRH. Thyroid hormones (T3/T4) will inhibit both the hypothalamus and the anterior pituitary
True or false… the thymus is the largest gland in the body
False. The thyroid gland is the largest gland in the body
True or false… the thyroid gland is poorly vascularized
False, it is extremely well vascularized
What is the difference between inactive and active thyroid glands in regards to…
Colloid
Follicle size
Cells lining the follicles
Colloid in the inactive gland is more abundant
Follicles are larger in the inactive gland
Cells lining the follicles of inactive glands are flat (cuboid in active)
The edges of follicles of active glands are scalloped with many small resorption gaps resulting form the uptake of colloid
The follicular cells of the thyroid secrete ____ and ____. The parafollicular cells secrete ____
Thyroxine (T4) and Triiodothyronine (T3)
Calcitonin
Thyroid hormones stimulate enzymes involved with ____ thus increasing _____ rates and _____ production
Glucose oxidation
Basal metabolic
Heat
Calcitonin is released in response to..
High blood calcium levels
How does calcitonin lower blood calcium levels?
By inhibiting the resorption of bone by osteoclasts
Name the three key hormones in regulation of blood calcium levels and their effect on calcium levels
1, 25 dihydroxycholecalciferol - formed from vitamin D and increases calcium levels
Parathyroid hormone - secreted by chief cells of parathyroid glands - increases blood calcium levels by stimulating osteoclasts
Calcitonin - decreases blood calcium levels by inhibiting osteoclasts - secreted from parafollicular cells of the thyroid
Thyroid hormones are synthesized from ____ and ____
Iodine and tyrosine
The thyroid gland produces small amounts of ____ but primarily secretes _____
T3
T4
T4 is converted to T3 in what extrathyroidal organs? What percent does each organ convert?
Liver - 60%
Intestines - 20%
The other 20% is inactivated by converting T3 to rT3 (most of this occurs in the liver)
What is rT3?
The inactive form of T3. It is an isomer of T3
Which is more potent, T3 or T4?
T3 (4x more potent)
What enzyme converts T4 to T3?
5’ de-iodinase enzyme
How many iodine groups are on T4? How about T3? How about rT3?
4
3
3
True or false… feedback inhibition at the level of the hypothalamus is the most efficient way to inhibit TRH secretion
False… feedback inhibiton at the level of the ANTERIOR PITUITARY is the most efficient way to inhibit TRH secretion
Are thyroid hormones lipophillic or hydrophilic? This means that they target ____ receptors
Lipophilic.
Nuclear
Under stress or fasting, the body converts (more/less) T4 to T3 and more T4 to ____ to conserve energy
Less
RT3
Explain how insufficient iodine levels can result in a goiter
Without sufficient iodine levels, thyroid hormones are not released/produced and TSH builds up. The increased levels of TSH will cause the thyroid to experience oxidative stress
True or false… thyroid hormone precursors can be stored in vesicles
True. Although the hormones are lipophilic, the precursers can be stored in vesicles
Are the half lives of thyroid hormones long or short?
Long
What is thyroglobulin? Explain its role in hormone production
It is stored in the follicle cavities of thyroid glands. They contain four to eight molecules of T3, T4, or both. Thyroglobulin serves as a reservoir for receiving the dietary iodine. Thyroglobulin is endocysosed by follicular cells and digested by lysosomes to release the thyroid hormones