Symptoms of low thyroid-stimulating hormone, or TSH, levels due to hyperthyroidism include rapid heartbeat, sudden weight loss, nervousness, tremors and increased appetite, notes Mayo Clinic. Some symptoms of low TSH levels due to secondary hypothyroidism are fatigue, depression, weight gain and cold intolerance, states UCLA Endocrine Surgery.
Testing TSH levels is the most reliable way to screen for conditions related to the thyroid gland, according to WebMD. When the pituitary gland releases TSH, the thyroid gland is triggered to produce the hormones triiodothyronine, or T3, and thyroxine, or T4, both of which help control metabolism and are necessary for proper brain growth during the early years of life. When an individual has an overactive thyroid gland, the pituitary gland stops releasing TSH, which is not needed to stimulate the thyroid to produce further T3 and T4, resulting in low blood TSH levels, explains Endocrine Web.
Some potential causes of hyperthyroidism are Grave's disease, noncancerous tumors called toxic nodules and toxic multinodular goiter, notes WebMD.
In some cases, low TSH levels are not indicative of hyperthyroidism but, rather, a damaged pituitary gland that cannot produce TSH, according to WebMD. This condition is called secondary hypothyroidism. Low TSH levels also sometimes occur in pregnant women during their first trimester and patients who take too much medication prescribed for an underactive thyroid gland.