How is hypothyroidism diagnosed?
Measurement of serum TSH is generally considered the best screening test for thyroid disease. Normally, the pituitary gland will secrete TSH in response to low thyroid hormone level (T4 free and/or T3 free). Thus, an elevated TSH level beyond the normal reference range would typically suggest an underactive thyroid.
Normal range differs from one lab to another and from one continent to another. In France, TSH is considered normal between 0.5 and 5.0 mIU/L (TSH lower than 0.5 indicates hyperthyroidism, higher than 5.0 indicates hypothyroidism). In Israel, normal is between 0.3 and 5.4 with variations, depending on the lab, such as 0.3 to 4.5. In USA, up to 2002, normal range of TSH level was 0.5 – 4.5.
The elevation of TSH is thought to reflect the sensitivity of the hypothalamic-pituitary axis to inadequate thyroid hormone level (decreased levels) in the blood which would be consistent with hypothyroidism. But its sensitivity creates a dilemma. As the thyroid gland fails, some patients are found to have elevated serum TSH levels, suggesting hypothyroidism, but have normal levels of free T4 and free T3 thyroid hormones (referring to subclinical hypothyroidism) – those hormones have fallen only slightly and are still within the normal range.
There is no question that an elevated TSH beyond 4.5 or 5.0 mIU/L will diagnose hypothyroidism, but the vast majority of patients who have hypothyroidism have TSH level below this limit; the diagnosis will be missed and they will remain untreated. Thus, a normal TSH does not exclude hypothyroidism. Even though the blood test shows normal levels, it does not, however, mean that the thyroid is functioning properly or is dealing adequately with the metabolism regulation. TSH often lags behind what is really going on. And what seems normal for one individual is not necessarily normal to another. The needs in hormones are different for each of us.
TSH can be affected by factors including hypopituitarism, acute stress, extreme non-thyroidal illnesses, or the first trimester of pregnancy.
Thus, the basic problem that traditional medicine has with diagnosing hypothyroidism is the so called "normal range" of TSH which is far too high. TSH measures a hormone from the pituitary gland, not the thyroid. This test is a highly accurate measure of serum TSH but not of the height of circulating thyroid hormone levels.
Since 2002, the AACE (American Association of Clinical Endocrinologists) may have resolved the problem. AACE considers nowadays TSH normal levels between 0.3 and 3.0 mIU/L, and estimates therefore that 27 millions of Americans suffer from hypothyroidism instead of the 13 million with the previous norms.
In addressing treatment, I will return to this crucial issue which should take into account many hypothyroid patients far ignored due to a so-called "normal" TSH.
Thyroid tests should include a systematic way following blood tests: TSH, free T4, free T3, and two anti-thyroid antibodies (anti-thyroid peroxidase and anti-thyroglobulin). This is unfortunately not the case in practice.
One exception: hypothyroidism with a low TSH
A low TSH lab test in the presence of a low free T4 and symptoms to match hypothyroid can point to hypopituitarism. This kind of thyroid disease is known as "secondary" or "central" hypothyroidism. However, the TSH may be in normal range, as well, in cases of central hypothyroidism. A special test, known as the TRH stimulation test (Thyrotropin Releasing Hormone), can help distinguish if the disease is caused by a defect in pituitary-hypothalamus. This test requires an injection of the TRH hormone.