What is Thyroid? The Thyroid is an endocrine gland. What is an endocrine gland? An endocrine gland is a ductless gland, as compared to a gland with a duct, such as a salivary gland. For instance, a salivary gland delivers its secretion, saliva, through its duct to a local area: in this case, the oral cavity. Glands with ducts are called exocrine glands. As compared to an exocrine gland, an endocrine gland secretes its hormone directly into blood circulation, which then exerts its affects on distant organs in the body.
Location Of Thyroid
The Thyroid is present in your anterior neck, lying just below the Adam’s apple (thyroid cartilage), in front of the trachea (wind-pipe). It moves up with the act of swallowing. If enlarged, you can see it move up with swallowing. That’s why your doctor asks you to swallow while inspecting and palpating your thyroid gland.
Structure of Thyroid
The Thyroid gland is shaped like a butterfly. It has two lobes, right and left, with a midline bridge called the isthmus.
At the microscopic level, the thyroid gland consists of closely packed sacs called follicles. Each follicle is filled with a protein material called colloid. The wall of each follicle is comprised of a single layer of epithelial cells, called follicular cells. These are the cells that produce colloid as well as the thyroid hormones, T4 and T3. T4 is also known as Thyroxine and T3 is also known as Triiodothyronine.
Scattered in between the follicles is another kind of cell called parafollicular cells or C-cells. These cells produce a hormone called Calcitonin, which is involved in the regulation of calcium level in the blood.
Function of Thyroid
The primary function of thyroid gland is to produce two thyroid hormones, T4 and T3. The thyroid follicle is the basic functioning unit of the thyroid gland. Think of a follicle as a sac, lined by a layer of thyroid cells. These are called follicular cells. These cells produce a special protein called Thyroglobulin, which is then stored in the central cavity of each follicle. Synthesis of thyroid hormones takes place on this preformed thyroglobulin.
Formation of Thyroid Hormones
Synthesis of thyroid hormones consists of three steps:
To synthesize thyroid hormones, thyroid cells need iodine, which primarily comes from diet. Iodine is a trace element which is present in variable amounts in the earth’s crust. However, sea water contains fairly good amounts of iodine. Dietary iodine is converted to inorganic iodide inside the body.
Each thyroid cell has an outer (basal) wall, an inner (apical) wall and two sidewalls. The basal wall actively transports iodide from blood circulation into the cell. This is called Iodide Uptake. Iodide is then transported inside the thyroid cell towards its inner apical wall, where synthesis of T4 and T3 takes place.
In order to produce thyroid hormones, the thyroid cell combines iodide with tyrosine, an essential amino acid present inside the thyroglobulin molecule. This process is called iodination of tyrosine. This chemical reaction is catalyzed by an enzyme called TPO (Thyroid Peroxidase) as well as H2O2 (Hydrogen Peroxide.)
As a result of iodination, two compounds are formed: MIT (Monoiodotyrosine) and DIT (Diiodotyrosine). Each MIT molecule contains one iodide atom and each DIT molecule contains two iodide atoms, attached to tyrosine.
The next step, called coupling, occurs when two DIT molecules fuse to form a molecule that contains four iodide atoms. This is called tetraiodothyronine or thyroxine or T4. Also, one MIT fuses with one DIT, which forms a molecule that contains three iodide atoms. This is called Triiodothyronine or T3. Only T4 and T3 are true thyroid hormones. MIT and DIT do not possess any hormonal activity.
Storage of Thyroid Hormones
After synthesis, MIT, DIT, T3 and T4 get stored in the thyroglobulin inside the lumen of the follicle. In this way, the thyroid gland serves as a large reservoir for storing the thyroid hormones. A normal thyroid gland stores about 8000 microgram of iodine, 90% of which is in the form of MIT, DIT, T3 and T4. The remaining 10% is in the form of iodide.
This unique storage function of the thyroid gland provides a safety-net against depletion of thyroid hormones, should synthesis ceases for some reason.
Release of T4 and T3
Small quantities of T3 and T4 are then released into blood circulation according to your body’s needs. This process involves re-absorption of thyroglobulin from the follicular lumen back into the thyroid cell, where thyroglobulin undergoes breakdown. Consequently, T4, T3, DIT and MIT are freed from the thyroglobulin molecule. T3 and T4 are released into blood circulation at the basal wall of the cell. MIT and DIT remain inside the cell and undergo further breakdown, as a result of which iodide is freed from tyrosine. A variable amount of freed iodide gets released into blood circulation. The remaining freed iodide stays inside the cell and is recycled for reformation of MIT and DIT.
Under normal circumstances, the thyroid gland releases about 80 – 90 micrograms of T4 and 6 – 8 micrograms of T3 per day.
Transport Of T4 and T3
Most of the T4 and T3 circulate in the blood, tightly bound to proteins, the most important of which is called TBG (Thyroid Binding Globulin). The other two less important binding proteins are TBPA (Thyroid Binding PreAlbumin) and albumin.
T4 and T3 in the bound form are metabolically inactive. Only a tiny fraction, 0.03% of total T4 and 0.3% of total T3, is present as Free T4 and Free T3 respectively. It is these free fractions that are available to tissues. Remember, even Free T4 is metabolically not very active. It needs to be converted to Free T3, which is the active thyroid hormone.
T4 to T3 Conversion in the Tissues
T3 is the active thyroid hormone, responsible for all of the biological actions of the thyroid hormone. Daily total production of T3 is about 32 micrograms (mcg), about 75-80% (24-26 mcg) of which comes from T4 to T3 conversion in the peripheral tissues. However, about 20-25% (6-8 micrograms) of the total daily production of T3 comes directly from the thyroid gland.
T4 to T3 conversion takes place under the guidance of an enzyme called 5′-deiodinase (DI). There are two types of 5′-DI: Type 1-DI and Type 2-DI. Type 1-DI is most abundant in the peripheral tissues, especially in the thyroid, liver, kidneys and muscles. Type 2-DI is mostly found in the brain and pituitary gland.
T4 Conversion to Inactive Reverse T3
T4 is also converted into an inactive form of T3 which is called reverse T3 (rT3). This conversion takes place under the guidance of another enzyme, called 5-deiodinase or Type 3 DI. Daily production of rT3 is about 19 micrograms
Metabolism Of T4 to other Inactive Compounds
About 70% of circulating Free T4 is converted to Free T3 and rT3 in a ratio of about 60% T3 to 40% rT3. The remaining 30% of Free T4 is converted into Inactive compounds through mechanisms independent of deiodinases. These mechanisms are sulfation, glucuronidation, deamination and decarboxylation of T4, primarily in the liver. These are called the alternative pathways of T4 degradation.
These alternative pathways of T4 degradation become clinically important when someone is on high dose of Levothyroxine. Typically, these are the patients who have undergone total thyroidectomy for thyroid cancer. A high dose of Levothyroxine causes a shift to increased T4 degradation into inactive compounds through alternative pathways. Consequently, less T4 is left for conversion into T3 (1). Therefore, these individuals suffer from a Low T3 state due to two reasons: a marked decrease in the peripheral T4 to T3 conversion and a lack of about 20% of daily production of T3 from the thyroid gland itself.
Deiodination means removal of one iodide atom. Each T4 molecule contains 4 atoms of iodide located at 3,3′, 5 and 5′ positions inside the molecule. Each T3 molecule contains 3 iodide atoms, located at 3, 3′, and 5 positions. Each Reverse-T3 molecule contains 3 iodide atoms, located at 3, 3′, and 5′ positions. T3 and Reverse-T3 are further deiodinated to T2, which is then deiodinated to T1 and ultimately to T0. Clinically speaking, T2, T1 and T0 are believed to contain no significant biologic activity.
T3 is the Active Thyroid Hormone
Out of all of the thyroid hormones, T3 is the most active hormone. In order to carry out the thyroid hormone function, T3 combines with Thyroid Hormone Receptor (THR) located inside the nucleus of a cell. T3 exerts its effects on almost every organ in the body, in particular the heart, brain, muscles, bones, skin, intestines and reproductive organs.
Regulation of Thyroid Hormone
The function of the thyroid gland is regulated in several ways:
- The Pituitary Gland regulates thyroid hormone production by producing TSH (Thyroid Stimulating Hormone). The Pituitary Gland senses the level of T3, judges it to be normal, low or high and produces the amount of TSH in an inverse In this way, the pituitary produces more TSH if T3 is low, and less TSH if T3 is high.
TSH goes to the thyroid gland and tries to increase or decrease the production of thyroid hormones: high TSH increases and low TSH decreases the production of thyroid hormones.
- The function of the pituitary gland is regulated by another endocrine gland, called the hypothalamus, which is located above the pituitary gland. The hypothalamus regulates the function of the pituitary gland by producing a number of hormones. In terms of thyroid regulation, it produces a hormone called TRH (Thyrotropin Releasing Hormone), which fine-tunes the production of TSH, which in turn regulates the production of thyroid hormones by the thyroid gland.
The hypothalamus itself is influenced by the limbic system, as well as various chemicals (neurotransmitters) in the brain. The Limbic system is the center of our emotions. In this way, stress as well as psychiatric illnesses as well as medications can affect the production of your thyroid hormones, by influencing your hypothalamus and pituitary gland.
- The Thyroid also has an incredible auto-regulation. For example, if there is an acute load of a large amount of iodine/iodide, the thyroid gland gets saturated with the amount of iodine it needs. Subsequently, there is a decrease in the amount of further uptake of iodine/iodide for a few days, after which uptake of iodine resumes normally. A large dose of iodine/iodide also temporarily decreases the release of thyroid hormones into circulation.
These are primarily protective mechanisms against the production and release of excessive thyroid hormones in case there is a sudden supply of large quantities of the raw material, iodine/iodide. (For example, contrast agents used during CT scans and angiograms contain huge quantities of iodine). In the same way, cough syrups usually contain large quantities of iodine. Iodine is also used as an antiseptic for skin cuts and wounds. Thanks to the auto-regulatory mechanisms, the vast majority of people do not become hyperthyroid or hypothyroid after a large load of iodine.
However, hypothyroidism or hyperthyroidism may rarely develop due to the chronic use of iodine in large doses. This has been reported in individuals with pre-existing Hashimoto’s thyroiditis or Graves’ disease. However, the vast majority of people do not become hyperthyroid or hypothyroid if they consume large amounts of iodine. The Japanese are the best evidence in this regard. The average Japanese consumes more than 12,000 micrograms of iodine per day, as compared to the typical American who consumes only about 240 micrograms of iodine per day. The incidence of hyperthyroidism or hypothyroidism is not significantly different between these two populations.
Excerpts from my book,”Hypothyroidism And Hashimoto’s Thyroiditis”