What is a Normal TSH Level?

What is a Normal TSH Level?

by W. Reid Litchfield, MD FACE

TSH is critical in diagnosing thyroid disorders, as well as in the evaluation of the adequacy of therapy in patients with thyroid disease. Controversy exists with respect to what represents normal versus abnormal TSH.

TSH is generally considered critical in determining when thyroid hormone replacement is optimal. It is also an essential tool in the diagnosis of hypothyroism. In the vast majority of cases, TSH levels are high in patients with hypothyroidism and drop as thyroid hormone replacement therapy is initiated. However, there has been controversy as to what the normal range for TSH is in healthy patients.

For many years the reference range for ultrasensitive TSH has been listed as 0.5 to 4.5 mIU/L. This reference range was obtained by measuring TSH in large numbers of patients with normal thyroid function and using statistical techniques to determine a reference range for the rest of the population. However, there is mounting evidence that some of the "normal" individuals in these surveys had unrecognized hypothyroidism with high TSH levels that skewed the results of the ‘normal’ TSH reference range. Subsequent scientific surveys of individuals that had been more rigorously tested to ensure normal thyroid function show that over 95% of the normal population will have a TSH between 0.4 and 2.5 mIU/L. It is therefore possible that some patients are told their thyroid function tests are "normal" based on an inaccurate reference range for TSH. This may potentially delay the diagnosis of hypothyroidism.

The scientific debate over this issue rages on. Some national societies such as AACE and ATA have issues statements in favor of more rigorous testing and possible treatment in symptomatic patients with TSH over 3 mIU/L (and appropriate confirmatory findings on more in-depth testing). However a combined consensus panel from AACE, Endocrine Society and ATA concluded that the upper limit of 4.5 mIU/L should not be lowered based on the lack of scientific evidence that intervening with thyroid hormone replacement in early thyroid failure has been proven to be beneficial.

Exposure to ionizing radiation is a well-known risk factor for thyroid cancer, but can also induce thyroid autoimmunity. Medical radiation (generally for cancer treatment) and radioactive fallout (Chernobyl nuclear accident, Hiroshima, nuclear testing) have all been linked to the development of anti-thyroid auto-antibodies and hypothyroidism. Radioactive iodine therapy has also been shown to cause the development of anti-thyroid antibodies and has been reported to cause Graves disease in some cases.

At the Nevada Thyroid Institute we see many patients with early thyroid failure when TSH levels still in the "normal" range that have symptoms that affect their day to day function and quality of life. Furthermore, we have seen many patients that notice a marked improvement in how they feel when they are given thyroid hormone replacement therapy and have their thyroid chemistry optimized. Admittedly, some patients with early thyroid failure and mildly elevated TSH are completely asymptomatic and clearly don’t warrant therapy.

Hopefully ongoing clinical trials will sort out this controversy. In the interim, we will continue to focus our medical decision-making based on the premise that we treat patients, not lab values. This means that for us the bottom line is that every patient needs to be considered as an individual. TSH levels are critical, but they are not the only factor. We must take into consideration other indicators of thyroid disease such as thyroid antibodies, Free T4 levels, physical examination of the thyroid gland and sometimes even thyroid ultrasound. Equally important is the patient’s history and description of how they feel-the very information that is most likely to be ignored by some physicians.

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