Thyroid stimulating hormone (TSH) levels evaluate the functioning of the thyroid gland. TSH instructs the thyroid to produce thyroid hormone. High TSH levels indicate that the thyroid is underactive, and a low TSH means the thyroid is overactive. Abnormal TSH levels alert medical professionals to diseases of the thyroid gland. The American Thyroid Association (ATA) recommends adults begin screening for thyroid dysfunction at age 35.
Normal TSH Levels
Laboratories report normal TSH levels between 0.5 and 5 mIU/L. Endocrinologists debate whether these values allow them to appropriately diagnose and subsequently treat patients suffering from thyroid dysfunction. The American Association of Clinical Endocrinologists (AACE) suggests that TSH levels between 0.3 and 3 mIU/L constitute a more normal range of thyroid function.
Some medications, stress and recent illness can affect TSH levels. Amiodarone, antithyroid medications, dopamine, lithium, potassium iodide and prednisone may interfere with TSH levels.
Patients can present with normal TSH levels but still experience symptoms of hypothyroidism or hyperthyroidism. If a patient's TSH falls in the normal range, thyroid tests for free T4 and free T3 and thyroid antibodies can rule out hypothyroidism and hyperthyroidism.
High TSH Levels
At a conference in September 2002, a panel representing the American Association of Clinical Endocrinologists, the American Thyroid Association and the Endocrine Society recommended that physicians adhere to the upper limits of 4.5 mIU/L as a guideline for treating subclinical thyroid disease. Still, many physicians adhere to the AACE clinical practice guidelines of 2002 recommendation of assessing for hypothyroidism, monitoring and possibly treating patients with TSH above 3 mIU/L.
High TSH levels can indicate congenital hypothyroidism, primary hypothyroidism, thyroid hormone resistance, TSH-dependent hyperthyroidism, or exposure to mice as seen in lab workers or veterinarians. Symptoms of an underactive thyroid can vary, but may include feeling cold, tired and depressed and being forgetful. Hypothyroidism can also cause weight gain, muscle pain and high cholesterol.
Low TSH Levels
Low TSH levels can indicate hyperthyroidism, TSH deficiency or the result of certain medications. Symptoms of an overactive thyroid include nervousness, irritability, increased perspiration, heart racing, hand tremors, anxiety, diarrhea, vision changes and muscle weakness.