The normal range for TSH (thyroid-stimulating hormone) levels can vary slightly among different laboratories, but most laboratories consider a normal range to be between 0.4 and 4.0 milliunits per liter (mU/L) for adults. However, a recent study suggests that the normal range should be more like 0.45 to 4.12 mU/L. It is important to note that pregnancy, a history of thyroid cancer, history of pituitary gland disease, and older age are some situations when TSH is optimally maintained in a different range as guided by an endocrinologist. Additionally, individuals taking medications that modify thyroid hormone metabolism and those with a history of thyroid cancer or pituitary disease may be optimally managed with a different normal FT4 range. It is important to consult with a healthcare provider to interpret TSH levels and determine appropriate treatment if necessary.