Each consists of a central, internal space called the lumen, surrounded by a wall of cells called follicular cells.
These cells change shape slightly from squamo-cuboidal to cuboido-columnar when stimulated to produce thyroid hormone
Parafollicular cell or C cells
a scattered group of cells surrounding each follicle.
What two hormones do thyroid follicles secrete?
Thyroxin (tetraiodothyronine, T4)
Triiodothyronine, T3
The prefix for each term describes the number of iodine molecules they contain, tetra- meaning four(T4) and tri-meaing three (T3)
Thyroid Function (part 1)
Thyroglobulin (TGB)is a protein synthesized inside the thyroid follicular cells.
TGB consists of 5000 amino acids, >100 of which are the amino acid tyrosine.
To synthesize the thyroid hormones, TGB is released into the lumen of the thyroid follicles.
Within the TGBmolecules, one or two iodine atoms attach to each tyrosine molecule.
Thyroid Function (part 2)
Tyrosin molecules link to form thyroid hormones containing three or our atoms of iodine.
When needed, TGB moves into follicular cells, and digestive enzymes cleave T3 and T4 from the TGB molecule.
T3 and T4 are lipid soluble, so they can diffuse through the plasma membrane and bind to the carrier protein thryroxin-binding globulin (TBG) in the blood.
How is thyroid hormone produced / secreted?
Low T3 and T4 levels or a low metabolic rate stimulates the hypothalamus to secrete thyrotropin-releasing hormone (TRH),
which results in the anterior pituitary producing thyroid-stimulating hormone (TSH).
TSH binds to TSH receptors in the follicular cells and activates a number of processes required to synthesize T3 N T4
What are the required steps for T3 and T4 synthesis?
Iodide trapping
synthesis of thyroglobulin (TGB)
oxidation of iodide
Iodination of tyrosine
Coupling of T1 and T2 to make T3 and T4
Pinocytosis and digestion of colloid
Secretion of thyroid hormones
Transport of T3 and T4 in the blood
Required steps for T3 and T4 synthesis:
1. Iodide trapping
Iodine circulates in the blood as iodide (I-). Iodide is actively transported into the follicular cells. Because of the process, the thyroid gland contains most of the iodide in the body.
Required steps for T3 and T4 synthesis:
2. Synthesis of thyroglobulin (TGB)
TGB is a glycoprotein produced by follicular cells. It contains large numbers of the amino acid tyrosine. Tyrosine is the site on the TGB molecule that will bind with iodine.
Required steps for T3 and T4 synthesis:
3. oxidation of iodide
Before iodide can bind to tyrosine, it must be oxidized and combine with another iodide to form an iodine molecule (I2).
Required steps for T3 and T4 synthesis:
4. Iodination of tyrosine
The side chain of tyrosine may pick up one (T1) or two (T2) iodine molecules.
Required steps for T3 and T4 synthesis:
5. Coupling of T1 and T2 to make T3 and T4
As pm pf tje ;ast ste[s. two tyrosine molecules are joined to form either T3 (T1+T2) or T4 (T2+T2).
Required steps for T3 and T4 synthesis:
6. Pinocygtosis and digestion of colloid
Once synthesized, the iodine-containing TGB reenters the follicular cells and digestive enzymes break down the molecule, releasing the formed T3 and T4.
Required steps for T3 and T4 synthesis:
7. Secretion of thyroid hormones
T3 and T4 are lipid soluble, so they freely pass the cell membrane into the interstitial fluid and into he blood.
Required steps for T3 and T4 synthesis:
8. Transport of T3 and T4 into the blood
Once in the blood stream, 99% of the secreted hormone binds to transport proteins, mainly thyrosine-binding globulin (TBG), not to be confused with TGB.
True or False
The thyroid gland is the only gland to store a large supply of its products.
True
approximately 100 days worth.
Thyroid sequence
Thyroid hormones
-Regulate oxygen use
-Increase the basal metabolic rate (BMR)
-increase protein synthesis
-Increase carbohydrate and fatty acid catabolism
-Increase reactivity of the nervous system by increasing sensitivity to ephinephrine and norephinephrine.
-Control tissue growth and development along with hGH.
What are the Thyroid hormone actions?
Increases the basal metabolic rate
Stimulates the synthesis of additional sodium-potassium (Na+/K+) pumps
Increases protein synthesis
Increases fatty acid and glucose catabolism
Decrease blood cholesterol
Increases the effects of epinephrine and norepinephrine
Accelerates body growth, especially during fetal life and adolescence.
Thyroid hormone actions:
Increases the basal metabolic rate
The basal metabolic rate is the rate fo oxygen consumption while awake, at rest, and fasting.
When the need for ATP increases, the use fo all nutrients increases.
Thyroid hormone actions:
Stimulates the synthesis of additional sodium -potassium (Na+/K+) pumps
this major action has a cascade of effects.
With the increase in Na+/K+ pumps, the demand for ATP is greater
As ATP is produced, calories are used, and more heat is produced (exothermic).
This is how thyroid hormones help a person regulate their normal body temperature.
Thyroid hormone actions:
Increase protein sysnthesis
Encourages growth
Thyroid hormone actions:
Increases fatty acid and glucose catabolism
Fatty acids and glucose are used to synthesize ATP
