The Thyroid

Functions

The thyroid gland is one of the most important glands as it secretes T4 (thyroxine) and T3 (triiodothyronine). These hormones influence metabolism and temperature, stimulate heart muscle, influence cellular activity, as well as increasing the use of fats, carbohydrates, protein, and cholesterol.  Thyroid hormones affect how your DNA is made and expressed.  The most important function of the thyroid is the regulation of mitochondrial function that makes ATP (a form of energy) that is required for almost all body functions to work well. In hypothyroidism (under-active thyroid), many bodily functions can become disrupted. Thyroid hormone helps control heart rate and blood pressure, and low thyroid can cause low heart rate, low blood pressure and abnormal heart rhythms.  Low thyroid function causes a sluggish metabolism and is presented as fatigue, depression, anxiety, weight gain, constipation, dry skin, irregular uterine bleeding, cholesterol imbalance, headaches, cold sensitivity, infertility, hair thinning, muscle pain, and memory changes. Hyperthyroidism would create opposite symptoms such as elevated heart rate, weight loss, sweating, palpitations, panic attacks, insomnia, and fatigue.

Physiology

The thyroid gland sits at the base of the front of the neck and is part of the hypothalamus-pituitary-thyroid “system” or axis. The hypothalamus is a gland in your brain that controls your endocrine system (adrenals, thyroid, ovaries, testes).  The hormone system functions on a feedback system, meaning the signals are sent from the body to the hypothalamus to release more hormones or to stop releasing hormones based on what is happening in the body.  Regarding the thyroid, the hypothalamus receives signals from throughout the body (ex: light, temperature, stress) to send out a signal to the pituitary to secrete thyroid stimulating hormone (TSH). The TSH now stimulates the thyroid to make thyroxine (T4) and triiodo-thyronine (T3).

The T4 (inactive hormone) must be converted to T3 (active hormone).  A small amount of conversion takes place in the thyroid (about 8%) and the rest of the conversion takes place throughout the body. There are three different enzymes (D1, D2, D3) that affect how T4 is converted to T3, and these enzymes are present in different tissue in the body.  The TSH (signal from the pituitary) has a different enzyme than the rest of the body and, therefore just a TSH lab, is not sufficient to determine if there is a problem with how your thyroid is functioning.  Have you ever been told there is nothing wrong with you, but you just do not feel good?  The TSH may go up and down, based on what the thyroid is doing, and when the thyroid becomes more dysfunctional, then the TSH will become more abnormal.

A few side notes

The last thing I will say about the thyroid function is how T4 is made and converted to T3.  One way the thyroid will conserve energy, if it receives a signal from the brain there is stress somewhere in the body, is to convert the T4 to RT3 (reverse T3) and this form is not available to the body.  The is one of the many ways the body tries to keep things stable.  I will refer to these when describing thyroid dysfunction. 

Conversion of thyroid hormone

Thyroglobulin is a protein and is the precursor to T4.  Thyroglobulin is created when triggered by the hypothalamus and requires many minerals and amino acids to support this process.  Once T4 has been created, then it must be converted to T3.  In the thyroid and throughout the body, the conversion of T4 (inactive) to T3 (active) is facilitated by thyroid perioxidase (TPO).

Causes of dysfunction

There are a few genetic mutations that affect the hypothalamus or pituitary, but these are rare and will not be discussed here.  There are some genetic variants that can make you more vulnerable to thyroid dysfunction.  Some genetic variants are associated to autoimmune and some can affect how the inactive hormone become active. (See the section on autoimmune).  The thyroid gland is dependent on several minerals and amino acids so gut dysfunction that affects absorption of your food can prevent the thyroid gland from getting the nutrients it needs to function. Stress is a contributing factor in many cases as the increase in cortisol (released with stress) sends a signal to the hypothalamus to slow things down, thus reducing the conversion of the inactive to active thyroid hormone. Acute dieting (strict reduction of calories that are too low) is a stressful event and the body responds by trying to conserve your energy to survive (reduce the conversion of inactive to active thyroid).  Anything that is causing intense stress, such as infection, toxins, vigorous exercise, obesity, and trauma can signal the hypothalamus to conserve energy by reducing the T4 to T3.

Many people affected by thyroid disease are unaware of their condition. Thyroid disorders affect more women than men due to the many hormonal changes throughout a woman’s life (birth control pills, pregnancy, menopause, menses). Age is a factor due to poor malabsorption in the older population.  Symptoms of menopause symptoms and hypothyroidism often overlap, but hormone balance cannot be achieved without correcting the thyroid imbalances.