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Thyroid gland


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What is the basic structure of the thyroid gland?
-2 lobes, seperated by an isthmus.
-Supplied with lots of blood.
-SNS and PNS innervation
What are the main functional units of the thyroid?
How are Thyroid follicles organized?
1. Follicular cells at periphery, w/ tight junctions between.
2. Filled with colloid
3. Surrounded by capillaries
4. Parafollicular cells btwn.
What is colloid?
The main ingredient in follicles; thick/gelatinous; composed of 10-25% thryoglobulin.
what is Thyroglobulin?
the storage form of thyroid hormones - 10-25% of colloid, so the thyroids possess their hormone in largely stored amts.
What are Parafollicular cells?
cells that secrete calcitonin
What is the main stimulus for release of TRH from the hypothalamus?
Cold temps
what two hormones of the hypothalamus affect thyrotrophs?
What are thyrotrophs?
2 hormones: TRH and somatostatin
Thryotrophs are the cells that secrete thyroid STIMULATING hormone in the PITUITARY.
how does thyroid stimulating hormone effect the thyroid follicles?
It increases their size, number, and function.
and what is the function of thyroid follicles?
To produce thyroid hormones.
How does thyroid hormone feedback on the hypothalamus?

How does TH effect Thyrotrophs?
-Inhibits TRH
-Stimulates Somatostatin
(both from hypothal)
-Inhibits TSH from pituitary
What is another name for TSH?
what ARE thyroid hormones, structurally?
Iodinated derivitives of Tyrosine (an amino acid).
Therefore, what does synthesis of TH rely on?
Iodine in diet - iodized salt
What two precursors are Thyroid hormones formed from?
What 2 THs are formed?
MIT and DIT. Each then forms T3 and T4 resp.
What are the ratios of T3 and T4 formed, which is more potent?
T3 is about 1-2% of TH in plasma; T4 is 98-99%; T3 is way more potent so you need less.
How is Iodide taken up by thyroid follicle cells from ECF?
-By active transport (ATP depnd)
-Used to synthesize Thyroglobulin
What occurs at junction between follicular cells and the colloid space within the Thyroid gland?
Tyr amino acids get iodinated inside Thyroglobulin, then form T3 or T4 inside the colloid lumen. Catalyzed by TPO enzyme.
Where are T3 and T4 stored until needed by the body?
In the lumen of the follicle - where colloid is.
How do the thyroid hormones get out of the follicles when needed by the body?
Taken out of lumen by endocytosis into the follicle cells, then lysosomes break down Thyroglobulin into amino acids and T3/T4. They diffuse into capillaries around thyroid.
Knowing the process of making T3 and T4, how does TSH affect TH synthesis and secretion?
By upregulating the enzymes that catalyze the processes, increasing Iodide uptake, Thyroglobulin synthesis, Tyr iodination, endocytosis, and Increasing thyroid follicle cell size tremendously!
why is TSH synthesis and secretion so complex?
because TH can't be stored in vesicles due to being hydrophob. This gives a mechanism for storing a reserve when iodine is unavailable.
what carries thyroid hormone in the SERUM? where is it synthesized?
Thyroxine binding globulin
-Synthesized in the liver.
what are the 3 plasma proteins for transfer of thyroid hormone?
-Thyroxine binding globulin 80%
-Albumin 10%
-Transthyretin 10%
Is there typically more bound or free thyroid hormone in plasma?
Bound - 99%; only 1% is free.
What is the purpose of Thyroid binding globulin?
It buffers the free thyroid hormone in our blood; prevents it from getting degraded too fast. If we don't need it, it's bound; if we do, it's let loose.
What is the primary effect that thyroid hormone has on cells?
It changes gene expression.
What form of thyroid hormone is the active form in tissues?
So how is T4 converted to T3?
by removal of iodine, by action of 5'-Deiodinase.
How is T4 converted in the
-Thyroid, liver, kidneys?
Brain: cells take up T4 and deiodinate in the cytoplasm.

Other: use 5'deiodinase to convert T4 to T3
How is T3 inactivated?
with 5-deiodinase
***(NOT 5'-deiodinase)***
It generates reverse T3, which is inactive, as opposed to normal T3 which is active.
What are the 3 main functions of Thyroid hormones?
1. CNS development
2. Growth
3. Basal metabolic rate regulation
When does thyroid hormone affect CNS development
IN the 3rd trimester to the 1st year of life.
How does TH affect growth?
-Increases GH synthesis
-Increases synthesis of structural and metabolic proteins in heart/muscle/liver.
How does TH affect the basal metabolic rate?
-Controls enzyme production;
-Incr. oxidative phosphorylation
-Incr. Heat production
What are 3 types of Hypothyroidism?
1. Cretinism
2. Goiter
3. Myxedema
What are Cretinism symptoms?
-Mental retardation due to impaired CNS
-Decreased body size
What is a Goiter?
An enlarged thyroid
What causes a goiter?
High levels of TSH - they increse follicle size signific!
-But you have low T3/T4 levels.
What are symptoms of "Myxedema"?
Can't stand cold temps
Bradycardia, edema in face. Droopy looking
What causes hypothyroidism?
Low iodine in the diet.
What is Hashimoto's disease?
An autoimmune destruction of the thyroid.
What are 2 categories of Hyperthyroidism?
1. Thyrotoxicosis - high TH levels from any source

2. Hyperthyroidism - just high TH levels from the gland.
What are symptoms of hyperthyroidism?
-Shaky, Nervous, fast heart rate, STARING, insomnia
Can a goiter be caused by hyperthyroidism?
Yes - from toxic T3 and T4 levels.
What type of goiter is associated with toxic levels of T3 and T4?
What are 2 causes of hyperthyroidism?
1. Grave's disease
2. Hamburger thyrotoxicosis
What is grave's disease?
An autoimmune disease where Ab stimulates TSH receptors on the follicle cells - the major cause of most hyperthyrodoism.
What is hamburger thyrotoxicosis?
When you ingest thyroid tissue i n improperly butchered meat (GROSS!)
What's the major difference between Hashimoto's and Grave's disease?
Both are autoimmune, but in Hashimoto, antibody destroys thyroid tissue and suppresses.
Graves, Antibody mimics TSH and binds its receptor.

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