Summary of Hypothyroidism medications
Triiodothyronine, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate. The thyroid hormones, triiodothyronine (T3) and its prohormone, thyroxine (T4), are tyrosine-based hormones produced by the thyroid gland that are primarily responsible for regulation of metabolism. T3 and T4 are partially composed of iodine. The major form of thyroid hormone in the blood is thyroxine (T4), which has a longer half-life than T3. In humans, the ratio of T4 to T3 released into the blood is roughly 20 to 1. T4 is converted to the active T3 (three to four times more potent than T4) within cells by deiodinases (5'-iodinase).
Transcript for Hypothyroidism medications
Now, as treatment for hypothyroidism, we can use medications that boost thyroid hormone synthesis, or we can thyroid hormone analogues as a replacement.
There are 2 different thyroid hormones; triiodothyronine or T3, and thyroxine or T4. They’re two tyrosine-based, iodine-containing hormones that are secreted by the thyroid gland, which is located anteriorly in the neck and consists of two lobes that look like two thumbs hooked together in the shape of a “V”.
Now, if we zoom into the thyroid gland, we’ll find thousands of follicles, which are small hollow spheres whose walls are lined with follicular cells or thyrocytes.
Zooming in, these follicular cells have an apical side that surrounds a central lumen filled with a viscous fluid called the colloid.
The colloid contains the precursor hormone thyroglobulin.
The basolateral side of follicular cells is in contact with blood vessels that supply these cells.
Now, synthesis of thyroid hormones inside the follicles involves a few important steps. First, the inorganic iodide ions, present in a low concentration in the blood, are actively taken up by the basolateral side of the follicular cells, along with two sodium ions, via a sodium- iodide symporter. This step is known as ‘iodide trap’.
The iodide ion is then pumped into the colloid via the pendrin protein, where it undergoes oxidation with the enzyme “thyroid peroxidase” or TPO, which changes it into an organic iodine atom.
It’s then attached to tyrosine amino acid residues which are found throughout thyroglobulin. This step is known as iodination.
Some tyrosine residues are bound by only one iodine, whereas others are bound by two iodine atoms, yielding monoiodotyrosine or MIT, and diiodotyrosine or DIT, respectively.
These molecules are then coupled together by the same enzyme “thyroid peroxidase” or TPO. This process is known as coupling.
Coupling one MIT with one DIT creates T3, while linking two DIT molecules creates T4.
In general, T4 is created in greater amounts than T3.
T3 is the more active form with a half life of one to two days, while T4 is the less active form with a longer half life of six to eight days.
Now, production and secretion of thyroid hormones is under the control of the hypothalamus- pituitary axis.
The hypothalamus, located at the base of the brain, secretes thyrotropin releasing hormone, or simply ΤRH, which stimulates the anterior pituitary cells called thyrotroph cells, to release the thyroid stimulating hormone, or TSH, into the bloodstream.
TSH then travels to the thyroid gland, and binds to the TSH receptors located in the membrane of the follicular cells of the thyroid gland.
When TSH binds to the TSH receptor, it goes on to promotes every aspect of T3 and T4 production, ranging from the iodide trapping to the release of thyroid hormones into the bloodstream.
Once released from the thyroid gland, T3 and T4 enter the circulation and travel via the blood by binding with the thyroxine binding globulin, or TBG, to reach the target cells.
Now, once inside the cell, T4 is mostly converted into T3 by the enzyme 5’- deiodinase.
T3 binds to thyroid hormone receptors which are within the cell’s nucleus, and these receptors regulate gene expression, which ultimately lead to various metabolic and physiologic effects in the body.
For example, T3 speeds up the cell’s basal metabolic rate, especially when we have to adapt to the environment.
So as an example, exposure to cold weather stimulates the release T3 and T4, and they go on to increase body metabolism and heat production by burning up more energy in the form of sugars and fats.
Thyroid hormones help activate the sympathetic nervous system, which is responsible for the fight or flight response. This increases heart rate which in turn increases the cardiac output, respiratory rate, and mental alertness.
Thyroid hormones also increase the gastrointestinal motility and they also have a very important growth and developmental role, and are necessary for normal neuronal development in growing fetuses and young children.
There are two types of hypothyroidism - primary and secondary.
In primary hypothyroidism, the thyroid gland is the problem, because it isn’t making enough thyroid hormones.