Causes of low thyroid-stimulating hormone, or TSH, levels include hyperthyroidism; a damaged pituitary gland that is unable to make TSH, called secondary hypothyroidism; and taking excess thyroid medicine when treating an underactive thyroid gland, explains WebMD. Women may experience low TSH levels in the first three months of pregnancy.
Hyperthyroidism means the thyroid gland is overactive, which is caused by conditions such as a noncancerous tumor called a toxic nodule, Graves' disease and a toxic multinodular goiter, notes WebMD.
Patients suffering from severe stress or a chronic illness, or who are taking medications such as corticosteroids and heparin may produce test results that are inconclusive, according to WebMD. The thyroid-stimulating hormone test is done to make sure the thyroid is functioning properly and to check the efficacy of thyroid treatments.