The thyroid is a butterfly-shaped gland in the lower neck that makes thyroid hormones that help regulate growth and development, body temperature, heart rate, and weight. Thyroxine (T4) and triiodothyronine (T3) are the two main thyroid hormones secreted into the bloodstream. The amount of thyroid hormones secreted is controlled by thyroid stimulating hormone (TSH), which is released from the pituitary gland in the brain. TSH stimulates the thyroid to secrete T3 and T4. When thyroid hormones are measured, the three most common markers are TSH, free T4, and free T3.
Thyroxine (T4) is the main hormone secreted into the bloodstream by the thyroid gland and it accounts for about 94% and the remaining 6% is triiodothyronine (T3). T3 is the most active form the body can use. T4 must be converted to T3 before the body can use it. Most of this conversion happens in the liver, but also take place in cells of the heart, muscle, gut, and nerves. These cells convert T4 to T3 with an enzyme, deiodinase which removes one molecule of iodine from T4. T4 has four iodine molecules and T3 has three iodine molecules. The majority of T4 is converted into T3 in the liver. About 20% of T4 is converted to T3 in the digestive tract. The rest of the T4 is converted into an inactive form, rT3 which the body cannot use. Levels of rT3 can become too high in times of major trauma, surgery, or severe chronic illness.
Hyperthyroidism is a condition in which the thyroid gland becomes overactive and produces too much thyroid hormones. An overproduction of thyroid hormones accelerates the body’s metabolism, causing unintentional weight loss and a rapid or irregular heartbeat.
Since the symptoms of hyperthyroidism are common among many other health conditions, diagnoses can be difficult. Symptoms typically include unintentional weight loss, rapid heartbeat, irregular heartbeat, increased appetite, nervousness, anxiety, sweating, increased sensitivity to heat, changes in bowel patterns, fatigue, skin thinning, and fine hair.
Causes: Graves’ Disease, thyroid nodules, and thyroiditis
Hyperthyroidism has multiple causes including Graves’ disease, thyroid nodules, and thyroiditis. Graves’ disease is an autoimmune disorder and is the most common cause of hyperthyroidism. The cause of Grave’s disease is unknown but most likely develops from a combination of genetics and environmental triggers such as from a virus. In Grave’s disease, the immune system produces an antibody called thyroid-stimulating immunoglobulin (TSI) that attaches to the thyroid cells and mimics thyroid-stimulating hormone (TSH). This causes the thyroid to overproduce thyroid hormones. The dysfunction of the immune system leading to the production of the antibodies, TSI, involves both B and T cells. Reduced inhibition of Th (T helper) cells is the main cause of Graves’ disease.
The inhibition of the Th cells is carried out by the suppressor T cells. The suppressor T cells are a subpopulation of T cells that modulate the immune system, maintain tolerance to self-antigens, and prevent autoimmune disease. They function by blocking the actions of some other types of lymphocytes to keep the immune system from becoming over-active. Research has found that a healthy liver is important to maintain the activity of the suppressor T cells. The suppressor T cells activity can be reduced in patients with compromised liver function such as hepatic cirrhosis.3
Thyroid nodules are benign fluid-filled lumps that form in the thyroid. The cause of thyroid nodules is not understood but they are very common. According to the American Thyroid Association, about one-half of all individuals will have a thyroid nodule by the age of 60. Patients often won't know they have a thyroid nodule until it is discovered during a routine medical exam. In some cases, the tissue in thyroid nodules can produce excess amounts of thyroid hormone, therefore, causing hyperthyroidism.
Thyroiditis is the inflammation of the thyroid gland that can cause stored thyroid hormone to leak out of the thyroid. Initially, thyroiditis causes hyperthyroidism due to the excess thyroid hormone but after about three months the thyroid becomes underactive and leads to hypothyroidism. Hypothyroidism is a condition in which the thyroid gland does not produce enough thyroid hormone (please refer to the hypothyroidism section discussed below to learn more). There are different types of thyroiditis that can cause hyperthyroidism and then hypothyroidism. These include subacute thyroiditis, postpartum thyroiditis, and silent thyroiditis. Subacute thyroiditis is a condition in which the thyroid becomes enlarged and painfully inflamed. The cause is unknown but researchers believe it may be due to a virus or bacteria. Postpartum thyroiditis can develop after a woman gives birth. Silent thyroiditis is referred to as silent because it typically causes no pain and likely develops due to an autoimmune condition.
Hypothyroidism and Goiter
Hypothyroidism, also called underactive thyroid, is a condition in which the thyroid does not make enough thyroid hormones. Decreased levels of thyroid hormones slow down the body’s metabolism, causing unintentional weight gain, slowed heartbeat, and swelling of the limbs and facial puffiness.
Hypothyroidism and its symptoms develop slowly over time as the condition progresses. The symptoms vary from person to person but some symptoms are common. Decreased levels of thyroid hormones slow down the body’s metabolism causing unintentional weight gain, slowed heartbeat, fatigue, sensitivity to cold, joint/muscle pain, dry skin, fertility problems, depression, and a goiter. A goiter is an abnormal enlargement of the thyroid gland. In some goiter cases, thyroid hormones are not produced enough (hypothyroidism). However, in other goiter cases, thyroid hormones are overproduced (hyperthyroidism).
Causes: Hashimoto’s disease
Hypothyroidism has many causes including Hashimoto’s disease, thyroiditis, congenital hypothyroidism, removal of the thyroid, some medications. Hashimoto’s disease is an autoimmune condition in which the patient’s immune system attacks the thyroid leading to chronic inflammation of the thyroid gland and is the most common cause of hypothyroidism. The cause of Hashimoto’s is not fully understood but researchers believe it is a combination of genetics and environmental factors. White blood cells called lymphocytes accumulate in the thyroid and produce antibodies that attack and destroy the cells in the thyroid gland. This lymphoid infiltration of the thyroid gland characterized in Hashimoto’s includes both B and T cells. These antibodies typically include thyroid peroxidase and/or thyroglobulin autoantibodies. Once enough cells have become damaged, the thyroid can no longer make enough hormones to regulate body functions. Similar to the Graves’ disease, reduced inhibition of Th (T helper) cells due to reduced activity of the suppressor T cells plays a significant role.
Please refer to thyroiditis in the hyperthyroidism section above.
Low T3 Syndrome
Low T3 Syndrome is characterized by low levels of T3 with normal T4 levels and either low or normal TSH levels. Although T3 levels are low and patients exhibit similar symptoms to hypothyroidism, low T3 Syndrome is not caused by a thyroid gland problem. The problem occurs often in the steps of T4 to T3 and rT3 conversions. Conversion of T4 is carried out by three different deiodinases, D1, D2, and D3 in multiple tissues and organs but primarily in the liver, gut, skeletal muscle, brain and the thyroid gland. D1 and D2 convert T4 to T3 and degrade rT3. It mainly occurs in the liver and gut. D3, however, converts T4 to rT3 which is the inactive form and degrades T3 and T4. In adults, D3 is mainly found in the brain which is the major organ that is clearing the thyroid hormones.2
Most studies on low T3 Syndrome have been done on patients with acute life-threatening conditions and have found that the majority of these patients will develop low T3 Syndrome. D3 expression occurs in other organs and tissues but primarily in the liver and gut and is induced by an ischemic condition. A stress-induced blood flow reduction induces the expression of D3 to lower the T3 level. Such a response is a protective mechanism under the ischemic condition to preserve the body’s energy.
Recently, more research has been focused on studying low T3 Syndrome in patients with non-critical chronic illness. The prevalence of low T3 Syndrome is found very high among patients with chronic fatigue syndrome. Low T3 Syndrome is also very common in clinical practice especially among patients with emotional, psychological or physical stress. The stress-induced blood flow reduction to the liver and gut might be able to trigger D3 expression causing low T3 Syndrome in these patients.
According to TCM, the autoimmunity in the case of Graves’ disease and Hashimoto’s disease is a condition associated with Spleen Damp. The reduced suppressor T cell activity is associated with a Liver Yin deficiency.