The thyroid gland, located at the base of the neck just below the Adam's apple, regulates metabolism in the body. The thyroid produces two main hormones--thyroxine (T4) and triiodothyronine (T3)--that influence every cell in the body. The pituitary gland and hypothalamus control the production of these hormones. TSH levels depend on the amount of the two hormones in the blood--too much of the hormones cause a low TSH level.
Normally, the thyroid releases the right amount of thyroxine and triiodothyronine, but sometimes it produces too much of these hormones. Hyperthyroidism, or an overactive thyroid, occurs when the thyroid releases too much of its hormones over a short or long period of time.
Graves' Disease
An autoimmune disorder called Graves' disease, occurs when the body's immune system turns against its own thyroid gland. According to the Mayo Clinic, Graves' disease accounts for most cases of hyperthyroidism. The immune system produces antibodies that fight against foreign substances such as viruses or bacteria. In Graves' disease, the antibodies mistakenly attack the thyroid and attach to specific activating sites on the thyroid gland, causing an increase in hormone production. Therefore, an overproduction of thyroid hormone causes a low thyroid-stimulating hormone, or TSH level.
Growths on the Thyroid
A benign growth within the thyroid gland is called an adenoma. These noncancerous lumps cause an enlargement of the thyroid. They may present with no clinical symptoms, but if they do, symptomatic hyperthyroidism may result. In this case, they can produce excessive amounts of thyroid hormones, which cause a low TSH level.
Thyroiditis
An inflammation of the thyroid gland from a viral infection or other causes, results in the condition known as thyroiditis. The inflammation affects the excessive amount of the thyroid hormones normally stored in the gland, and results in the thyroid hormones leaking into the bloodstream. The pituitary gland produces low levels of TSH in response to the elevation of thyroid hormones.
Medication
Dr. James Norman, MD, FACS, FACE of Endocrineweb, reports that a low TSH level can also occur in patients who take excessive doses of the thyroid hormones thyroxine (T4) and triiodothyronine (T3) through prescribed medication.
