Thyroid deficiency is a common disorder where there is inadequate cellular thyroid levels to meet the needs of the tissues. Typical symptoms include fatigue, weight gain, depression, cold extremities, muscle aches, headaches, decreased libido, weakness, cold intolerance, water retention, premenstrual syndrome (PMS) and dry skin. Low thyroid function can cause or contribute to the symptoms of many conditions. Unfortunately, thyroid deficiency is often missed by standard thyroid testing. This is frequently the case with depression, hypercholesterolemia (high cholesterol), menstrual irregularities, infertility, PMS, chronic fatigue syndrome (CFS), fibromyalgia, fibrocystic breasts, polycystic ovary syndrome (PCOS), hyperhomocysteinuria (high homocystine), atherosclerosis, hypertension, obesity, diabetes and insulin resistance.
TSH traditionally has been thought to be the most sensitive marker of tissue levels of thyroid hormone. Despite this traditional thinking, newer information suggests that a normal TSH does not necessarily indicate that a person’s tissue thyroid levels are adequate. In fact, a more thorough understanding of thyroid hormone physiology demonstrates how TSH is NOT an accurate marker of the body’s overall thyroid status.
It is certain that TSH inversely correlates with pituitary T3 levels. However, physiologic stress, depression, insulin resistance and diabetes, aging, calorie deprivation (dieting), inflammation, PMS, chronic fatigue syndrome and fibromyalgia, obesity and numerous other conditions, are often associated with diminished cellular and tissue T3 levels – and increased reverse T3 levels. Thus, with physiologic or emotional stress, depression or inflammation, pituitary T3 levels do not correlate with T3 levels in the rest of the body. As a result, TSH is not a reliable or sensitive marker of an individual’s true thyroid status.
TSH is merely a marker of pituitary levels of thyroid function and not of thyroid hormone levels in any other part of the body. Only under ideal conditions of total health do pituitary thyroid hormone levels correlate with thyroid hormone levels in the rest of the body, making the TSH a poor indicator of the body’s overall thyroid status. With the above-mentioned conditions, most individuals with diminished tissue levels of thyroid hormone will have a normal TSH. In other words, the relationship between TSH and tissue thyroid hormone is lost in the presence of physiologic or emotional stress, depression, insulin resistance and diabetes, aging, calorie deprivation (dieting), inflammation, PMS, chronic fatigue syndrome and fibromyalgia, obesity and numerous other conditions. In the presence of such conditions, a normal TSH cannot be used as a reliable indictor that a person is euthyroid (normal thyroid) in the overwhelming majority of patients.
In the presence of such conditions, T4 levels also are not a reliable indicator of adequate thyroid function. These conditions lead to a suppression of the tissue’s ability to convert T4 into T3. Furthermore, there is an increased conversion of T4 to reverse T3 – an inactive form of T3 (a thyroid inhibitor for all practical purposes). AlthoughT4 levels are important, as with the TSH, the serum T4 level is often misleading and an unreliable marker of the body’s overall thyroid status.
With increasing knowledge of the complexities of thyroid function, it has become clear that TSH and T4 levels are not the reliable markers of tissue thyroid levels as once thought – especially with chronic physiologic or emotional stress, illness, inflammation, depression and aging. It is common for an individual with. normal TSH and T4 levels to complain of symptoms consistent with reduced thyroid function.
While there are limitations to all testing and there is no perfect test, obtaining TSH, free T4, free T3, reverse T3, and T3/reverse-T3 ratios can be helpful to obtain a more accurate evaluation of overall thyroid status – and these values may be useful to predict those individuals who may respond favorably to thyroid supplementation. Many symptomatic patients with normal TSH and T4 levels significantly benefit from thyroid replacement, often with significant improvement in fatigue, depression, diabetes, weight gain, PMS, fibromyalgia and numerous other chronic conditions.
With an understanding of thyroid physiology, it becomes clear why a large percentage of patients treated with T4 only preparations continue to be symptomatic. As discussed above, with any physiologic stress (emotional or physical), inflammation, depression, inflammation, aging or dieting, T4 to T3 conversion is reduced and T4 will be preferentially converted to reverse T3, which acts a competitive inhibitor of T3 (blocks T3 at the receptor), reduces metabolism, suppresses T4 to T3 conversion and blocks T4 and T3 uptake into the cell.
While a normal TSH cannot be used as a reliable indicator of global tissue thyroid effect, even a minimally elevated TSH (above 2) is a clear indication (except in unique situations) that the rest of the body is suffering from inadequate thyroid activity. Thus, treatment should likely be initiated in any symptomatic person with a TSH greater than 2. Additionally, many individuals will secrete a less bioactive TSH so for the same TSH level, a large percentage of individuals will have reduced stimulation of thyroid activity, further limiting the accuracy of TSH as a measure of overall thyroid status. Reduced bioactivity of TSH is not detected by current TSH assays used in clinical practice.
Due to the lack of correlation of TSH and tissue thyroid levels, a normal TSH should not be used as the sole reason to withhold treatment in a symptomatic patient. A symptomatic patient with an above average reverse T3 level and a below average free T3 (a general guideline being a free T3/reverse T3 ratio less than 2) should also be considered a candidate for thyroid supplementation.
