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Saturday, August 27, 2011

That Beautiful Butterfly in your Neck: The thyroid… What is it and what does it do?

That Beautiful Butterfly in your Neck: The thyroid… What is it and what does it do?

The thyroid gland is a butterfly-like structure in your neck. In men where the ‘Adam’s Apple’ is visible, the thyroid gland is right there. In women, where the ‘Adam’s Apple’ is not as visible, it is harder to feel where it is, yet it is right there, sitting low in the front of your neck. It is in front of the wind pipe (trachea) and has two side lobes, connected by a bridge (isthmus) in the middle. When the thyroid is normal size, you can’t feel it.

The thyroid gland is brownish-red in color, rich with blood vessels. Nerves important for voice quality also pass through the thyroid, so when the thyroid enlarges it may affect the voice (making it hoarse).
The thyroid gland is super important. It is subtle, and yet necessary for life. The thyroid secretes several hormones, collectively called thyroid hormones that act throughout the body. It is likely that all cells in the body are in some way or another related and need some degree of thyroid hormones. The thyroid and its hormones have profound effects on development, growth and metabolism, and a deficiency in thyroid hormones is not compatible with normal health.

As an adult, a properly functioning thyroid gland is necessary for life. This is quite the statement, yet after a couple of years of not taking thyroid hormone medication, an elderly woman may even die.

More specifically thyroid hormones have effects in the following systems and processes:

1. Development: Of critical importance in mammals is the fact that normal levels of thyroid hormone are essential to the development of the fetal and neonatal brain and brain function.

2. Growth: Thyroid hormones are clearly necessary for normal growth in children. We can confirm this by the observation of the delay in growth when there is a deficiency of thyroid hormones. Thyroid hormones appear to work with growth hormones and other endocrine hormones as a team.

3. Metabolism: Your metabolism – the rate at which you produce and use energy – is regulated by the pituitary and thyroid glands, and the hypothalamus (in the brain), but the actual energy release occurs inside the body’s individual cells. It is here that the thyroid’s role is literally felt. Each cell has an “energy-generating station” called the mitochondrion, which uses enzymes to combine carbon, hydrogen, and oxygen atoms, forming carbon dioxide and water, and releasing chemical energy. Thyroid hormones increase basal metabolic rate. As metabolism increases, there is an increase in body heat production. This increase in heat results in part, from increased oxygen and energy (ATP) consumption. A few examples of specific metabolic effects of thyroid hormones include:

a. Lipid metabolism: Increased thyroid hormone levels stimulate fat mobilization and enhance oxidation of fatty acids, leading to decreased cholesterol.
b. Carbohydrate metabolism: Thyroid hormones stimulate almost all aspects of carbohydrate metabolism, including enhancement of the entry of glucose into the cells by allowing insulin to work better. Thyroid hormones also increase the liver’s ability to generate free glucose.

4. Cardiovascular system: Thyroid hormones increases heart rate, cardiac contractility and cardiac output. They also promote vasodilation, which leads to enhanced blood flow to many organs.

5. Central nervous system and psychology: Both decreased and increased concentrations of thyroid hormones lead to alterations in mental state. Not enough thyroid hormone may cause a person to feel mentally sluggish, while too much may cause anxiety and nervousness.

6. Reproductive system: Normal reproductive behavior and physiology is dependent on having essentially normal levels of thyroid hormone. When the thyroid hormones are off, a woman may have menstrual irregularities. Not enough thyroid hormone, such as in the case of hypothyroidism, is commonly associated with infertility. Too much thyroid hormone, hyperthyroidism, is associated with frequent miscarriages.

How does the thyroid work?

The thyroid gland is under the influence of the brain. In the brain a specific area called the hypothalamus sends a message to another very important area the pituitary gland. This messenger hormone is called Thyrotropin-Releasing Hormone (TRH), also called thyrotropin-releasing factor (TRF).

This hormone, TRH, stimulates the pituitary to release Thyroid-Stimulating Hormone (TSH) and prolactin.

TSH is a very important hormone. It gives us the clue of what is really happening when the thyroid gland is off. When the thyroid is not working right, the pituitary gland will react by either producing more or less of the TSH.

TSH is a glycoprotein and consists of two subunits, the alpha and the beta subunit. The α (alpha) subunit is nearly identical to that of human chorionic gonadotropin (HCG), luteinizing hormone (LH), and follicle-stimulating hormone (FSH). The β (beta) subunit is unique to TSH, and therefore determines its receptor specificity.

The thyroid gland, under the influence and command of TSH produces T1, T2, T3 and T4. The numbers are the number of iodine molecules on the thyroid hormone. So, T1 has 1 iodine molecule, T2 has 2, T3 has 3 and T4 has four iodine molecules. T3 is the most biologically active thyroid hormone. It is formed by combining a T1 with a T2 (so the total of iodides in the molecule is 3). T4 is formed in much greater quantity by combining a T2 with another T2 (so that the total of iodides in the molecule is 4). These hormones are then stored in the thyroid gland and released into the blood stream.

The level of thyroid hormones in the blood has an effect on the pituitary release of TSH; so when the levels of thyroid hormone are low, the production of TSH is increased, in order to stimulate the thyroid further and produce more thyroid hormone. On the other hand, when thyroid hormone levels are high, TSH production is decreased. This effect creates a regulatory negative feedback loop.

It is important to know this, because one of the most common tests that measure the thyroid and its function is the TSH. When it is HIGH it means the thyroid is not working well and thyroid hormone levels are LOW, and the opposite is true as well, when the TSH levels are LOW, it means that the thyroid gland is producing TOO MUCH thyroid hormone.
But TSH is not all. IODINE is needed.

To make thyroid hormone, the thyroid uses iodine. If iodine is not available in the diet, the thyroid may not be able to produce sufficient amounts of T3 and T4 hormones.

The recommended daily intake of iodine for adults is 150 micrograms/day, and for children it is 90-120 micrograms/day. Pregnant Women require more iodine and their recommended daily intake is about 200 micrograms/day.

The process by which the thyroid uses iodine is actually quite complicated and certain steps are still unclear. Essentially, iodine is converted to its free elemental form, called iodide. Iodide enters the thyroid gland through a special transport mechanism. Iodide then undergoes a process called oxidation and is incorporated into intermediate hormones T1 and T2. These compounds then combine to form the active hormones, tri-iodothyronine (T3) and thyroxine (T4) as I described earlier.

So in summary: thyroid hormone is actually made up of iodide/iodine directly. So you can see the importance of iodine in relation to the function of the thyroid gland.

It is important to understand the building blocks for thyroid hormone and what it does. The hypothalamus (TRH), the pituitary(TSH), the thyroid gland (T1, T2, T3, T4) and iodine are key factors in making sure all goes well. Once you understand this, then it will be easier to understand the different problems that commonly affect the thyroid, and cause thyroid disease in people.

In my next blog, we will review the problems that arise when the thyroid gland does not work well, commonly called HYPOTHYROIDISM.
I hope you enjoyed the info in the thyroid. Stay tuned….

Margarita Ochoa-Maya, MD

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