The thyroid helps control a number of metabolic functions, including temperature regulation, reproductive function, weight and heart rate, among others. Thyroid stimulating hormone, produced by the pituitary gland, tells the thyroid to increase production of thyroid hormones T3 and T4 much like a thermostat tells the furnace to turn on when heat levels drop. When TSH levels are low, T3 and T4 levels are usually high. Low TSH levels are usually associated with hyperthyroidism, which causes a number of symptoms. Symptoms of low TSH are not always easily recognized, particularly in the elderly.
Causes
Low TSH levels usually occur because the thyroid is producing larger than normal amounts of T3 and T4. Symptoms associated with low TSH levels are caused by the increased levels of thyroid hormones. The excess thyroid hormone production increases the metabolic effects that come under the thyroid's control.
Types
Low TSH levels can occur because of disease in the pituitary or the thyroid. If damage affects the pituitary, it can't manufacture TSH, so levels drop. More often, damage occurs to the thyroid, and it produces too much T3 and T4. Low levels of TSH from pituitary disease causes fewer problems than low TSH related to thyroid disease. Graves' disease, the most common cause of hyperthyroidism, occurs when the thyroid increases in size and produces large amounts of thyroid hormones. The pituitary turns off TSH production when thyroid hormone levels rise.
Symptoms
Metabolic processes speed up when TSH levels fall due to thyroid damage. Symptoms include fast heart rate, weight loss, increased appetite, increased sensitivity to heat and rapid peristalsis, or movement of waste, through the intestine, which often causes diarrhea. Fatigue, weakness and difficulty sleeping often occur, along with increased anxiety, nervousness, restlessness and irritability. Eyes may bulge outward and double vision or blurred vision may occur.
Treatment
The symptoms of low TSH levels are treated by medications to lower levels of T3 and T4, which then allows the pituitary to begin producing TSH again to control thyroid hormone production. Radioactive iodine shrinks the thyroid gland, while anti-thyroid medications block the amount of thyroid hormones produced by the thyroid. Medications can take several months to work and may need to be continued permanently. Anti-thyroid medications can seriously damage the liver, MayoClinic.com warns. Surgical removal of part of the thyroid is recommended for people who don't respond well to or who can't take anti-thyroid drugs or iodine. Between 2 and 16 percent of people who have surgery experience a recurrence of symptoms, The Merck Manuals Online Medical Library states.
Complications
Symptoms of low TSH and hyperthyroidism can lead to serious complications. Heart complications such as atrial fibrillation, a quivering of the atria, the top chambers of the heart, rather than a coordinated beating can reduce blood flow through the heart. Congestive heart may occur if blood backs up in the heart and lungs. Thyrotoxic crisis, a severe exacerbation of untreated or uncontrolled hyperthyroidism, causes high fever, weakness, psychosis and coma followed by cardiovascular collapse and shock Death can occur as a result.
