Term
| What is the major controller of TSH secretion? |
|
Definition
- This is thyroid-releasing hormone (TRH) - Is secreted by hypothalamic neurons into blood vessels that link the hypothalamus and the pituitary (Hypothalamic-hypophyseal portal) - TRH receptors are found on thyrotrophs in the anterior pituitary and stimulate secretion of TSH |
|
|
Term
| Where are TRH neurons found and how do they work? |
|
Definition
- TRH neurons are found in the paraventricular nucleus of the hypothalamus - TRH is released into the portal capillaries for transport to the anterior pituitary in the blood - TRH neurons receive input from metabolic, temperautre, reproductive neurons. |
|
|
Term
| Picture of Hypothalamus and Pituitary Gland Relationship |
|
Definition
|
|
Term
| Negative Feedback Loop of Thyroid Gland |
|
Definition
- Secretion of TRH, and hence, TSH, is inhibited by high blood levels of thyroid hormones - Thyroid hormones reduce the response of the pituitary thyrotrophs to TRH. - Thyrotrophs ability to respond to TRH depends on their intracellular concentration of T3, 80% of which is derived from the conversion of T4 to T3 - When levels of T4 are low, there is an increase in TRH receptors and in TSH synthesis, resulting in an increased TSH response to TRH. - In the presence of high circulating concentrations of thyroid hormones there is a reduction in TRH receptors and TSH synthesis is decreased. |
|
|
Term
| Carriers for Thyroid Hormones |
|
Definition
- Are poorly soluble in water, and more than 99% of the T3 and T4 circulating in blood is bound to carrier proteins; thyroxine-binding globulin, a glycoprotein synthesized in the liver. - Other carriers of importance are transthyrein and albumin. |
|
|
Term
|
Definition
- Thyroxine (T4) and Triiodothyronine (T3) are Iodinated tyrosine residues of thyroglobulin - Synthesis - Uptake of iodide by gland, oxidation and iodination. |
|
|
Term
|
Definition
- The thyroid hormones are basically two tyrosines linked together with the critical addition of iodine at three or four positions on the aromatic rings. The number and position of the iodines is important. [image] |
|
|
Term
| Uptake and Organification of Iodine by the Thyroid Gland |
|
Definition
|
|
Term
| Process of Iodine Uptake (listed steps) |
|
Definition
1.1. Active uptake of iodide (I-) in exchange for Na+.
2.2. Iodide may be discharged from the follicular cell by administration of competing ions such as perchlorate, bromide or chlorate.
3.3. Iodide uptake, the main control point for hormone synthesis, is stimulated by TSH.
4.4. Oxidation of iodide by hydrogen peroxide (H2O2) to form active iodine. The reaction is catalyzed by thyroid peroxidase (TPO).
5.5. Active transport of iodine across the apical surface of the follicular cell.
6.6. Incorporation of active iodine into the tyrosine residues of thyroglobulin molecules to form mono- and di-iodotyrosines (MIT and DIT).
7.7. Uptake of the thyroglobulin into the lumen of the follicle and lining of iodinated tyrosine residues.
8.8. About 1% of stored colloid is removed each day. When the gland is very active this may rise to nearly 100% and colloid stores are depleted. |
|
|
Term
| How does TSH affecdt T3 and T4, and how much of each is secreted each day? |
|
Definition
Ø1. Under the influence of TSH, colloid droplets consisting of thyroid hormones within the thyroglobulin molecules are taken back up into the follicular cells by pinocytosis.
Ø 2. Fusion of colloid droplets with lysosomes causes hydrolysis of thyroglobulin and release of T3 and T4.
Ø 3. About 10% of T4 undergoes mono-deiodination to T3 before it is secreted. 4. The released iodide is reutilized. Several-fold more iodide is reused than is taken from the blood each day but in states of iodide excess there is loss from the thyroid.
Ø 5. On average approximately 100 mg T4 and about 10 mg T3 are secreted per day. |
|
|
Term
| Hormone Distribution in Blood and Metabolism |
|
Definition
A. Free vs. Protein-bound (>99%) 1. Thyroxine-Binding Globulin (TBG) B. Plasma Half-Lives - very long 1. Deiodination in Liver and Kidneys |
|
|
Term
| Actions of Deiodinization |
|
Definition
ØNormal growth and development—brain development (Myelin basic protein) Ø Calorigenic effect—responsible for lipogenesis and thermogenesis ØCardiovascular effects—hyperthyroidism (tachycardia); hypothyroidism (bradycardia) Ø |
|
|
Term
| Effects of Deiodinization |
|
Definition
1. Calorigenic - Increases Basal Metabolic Rate 2. CNS development and Function 3. Growth |
|
|
