What’s normal can vary, depending on a number of factors, including the laboratory where your blood test is done, she adds. A normal range for TSH in most laboratories is 0.4 milliunits per liter (mU/L) to 4.0 mU/L.
If your TSH is higher than 4.0 mU/L on repeat tests, you probably have hypothyroidism.
Your doctor may also order a T4 test. Most of the T4 in your blood attaches to a protein, and when it does, it can’t get into your cells. Only T4 that is unattached or “free” can get into your cells to go to work. A blood test can measure how much free T4 is available.
It makes thyroid-stimulating hormone, or TSH, which tells the thyroid gland how much T4 and T3 to produce. The TSH level in your blood reveals how much T4 your pituitary gland is asking your thyroid gland to make. If your TSH levels are abnormally high, it could mean you have an underactive thyroid, or hypothyroidism