What Do TSH Blood Test Levels Determine?

TSH blood levels indicate the amount of thyroid-stimulating hormone in the blood. A well-functioning thyroid has a blood level of 0.4 to 4.0 milli-international units per liter, says MedlinePlus. A blood level below or above that range indicates a possible problem with the thyroid.

The TSH hormone, produced by the pituitary gland, signals the thyroid to release two more hormones, T3 and T4, to help regulate the body’s metabolism, WebMD asserts. A TSH blood level test gives an indication as to how well the thyroid is functioning. Physicians can order a thyroid blood test to see how well the T3 and T4 hormones function in the system, but the TSH blood level test takes an overall look at the thyroid, according to MedlinePlus.

A TSH blood level reading checks whether or not the thyroid is underactive or overactive. A TSH level higher than 4.0 indicates hypothyroidism, a condition in which the thyroid underperforms, states MedlinePlus. Symptoms of hypothyroidism include weight gain, depression and tiredness, reports WebMD.

A TSH blood level lower than 0.4 suggests that the thyroid is too active. This condition is called hyperthyroidism. Symptoms of hyperthyroidism that may merit attention include weight loss, nervousness, and diarrhea, says WebMD.