Testing Thyroid Function
Thyroid stimulating hormone (TSH). TSH level is the most common test for screening for thyroid dysfunction. In the last decade the diagnostic strategy for using TSH measurements has changed as a result of the sensitivity improvements in these assays. It is now recognized that the TSH measurement is a more sensitive test than T4 for detecting both hypo- and hyperthyroidism.59 As a result, some countries now promote a TSH-first strategy for diagnosing thyroid dysfunction in patients.60
In 2008 many labs adopted the reference range for TSH, 0.45 to 4.50 μIU/mL, recommended by both the Endocrine Society and the American Medical Association. Although this range is an improvement over the previous 0.45-5.5 mIU/L, it is still considered too broad by many clinicians.59,60,61
The American Association of Clinical Endocrinologists now recommends an upper limit of 3.0 mIU/L.61 The guidelines for diagnosing thyroid disease from The National Academy of Clinical Biochemistry point out that "more than 95% of normal individuals have TSH levels below 2.5 [µIU/mL]."62 This panel suggests that the upper limit of TSH should be reduced to 2.5 µIU/mL.63
On the other hand, current studies also suggest that TSH values below the normal range may represent thyroid hormone excess and, in elderly patients, might be associated with an increased risk of death due to cardiovascular disease.64,65
Life Extension suggests an optimal level of TSH between 1.0 and 2.0 µIU/mL, as some studies have noted that a TSH above 2.0 may be associated with adverse cardiovascular risk factors.26 In addition, a TSH between 1.0 and 2.0 µIU/mL has been associated with the lowest subsequent incidence of abnormal thyroid function.66
However, while a measure of TSH alone is a useful screening tool in assessing thyroid function, Life Extension advocates additional testing, including Free T3 and T4 levels, to provide a more complete evaluation of the thyroid.
Note: TSH values do fluctuate with time of day, infection, and various other factors. In a 2007 survey published in the Archives of Internal Medicine, values spontaneously returned to normal in more than 50% of patients with abnormal TSH levels when the test was repeated at a later date.67 No single measurement of TSH should be considered diagnostic.
Basal Body Temperature. An alternative method for assessing thyroid status that was widely used in the past, before the development of accurate thyroid function blood tests, is the basal body temperature test. The temperature is taken when the body is at complete rest, immediately after waking and before beginning any activity. The normal basal temperature is 97.6-98.2 ºF, and some alternative practitioners believe that a 5-day consecutive temperature reading below 97.6 ºF is indicative of hypothyroidism. One study showed a significant correlation between the basal body temperature and low thyroid function in whiplash patients. The authors of this study conclude that basal body temperature “seems to be a sensitive screening test, in combination with laboratory analysis, for the hypothyroidism seen after whiplash trauma.”68 However, there are many reasons for alteration of basal body temperature, a thyroid panel blood test should be taken to accurately evaluate the thyroid function.
Tests for T4 and T3. Thyroid hormones can be tested in both their free and protein-bound forms. Tests for the protein-bound forms and unbound form of T4 or T3 are generally referred to as Total T4 or Total T3 respectively; unbound forms are called Free T4 and Free T3. Each of these tests gives information about how the body is making, activating, and responding to thyroid hormone. Levels of free T3 and T4 will be below normal in clinical hypothyroidism. In subclinical hypothyroidism the TSH will be elevated while the thyroid hormone levels are still in the normal reference range.
Reverse T3. Certain individuals with apparently normal T4 and T3 hormone levels still display the classic symptoms of hypothyroidism. This may be due to an excessive production of reverseT3 (rT3). rT3 is inactive and may interfere with the action of T3 in the body. Stress and extreme exercise may play a role in lowering thyroid hormone action by suppressing production of TSH and T3 and elevating rT3 levels.69,70
Autoimmune antibodies. When evaluating the thyroid it is also important to consider that the most common cause of overt hypothyroidism in the United States is an autoimmune disorder known as Hashimoto’s thyroiditis.71 In this condition the body produces antibodies to the thyroid gland and damage the gland. Hashimoto’s thyroiditis is diagnosed by standard thyroid testing in conjunction with testing for the presence of these antibodies called antithyroglobulin antibodies (AgAb) and thyroperoxidase antibodies (TPOAb). Some people with celiac disease or sensitivity to gluten are at increased risk for developing autoimmune thyroid disease and should be evaluated.72
Elevated thyroid antibodies are often associated with chronic urticaria, also called hives. Studies report that as many as 57.4% of patients with hives have the presence of anti-thyroid antibodies.73,74 An August 2010 paper suggests that treatment with T4 improves the itching associated with urticaria, but did not advise treatment with T4 unless the patient was hypothyroid.75
Additional testing. Sometimes biopsy or enzymatic studies are required to establish a definite diagnosis for thyroid dysfunction. Major abnormalities of the thyroid gland detected in physical exam can be further assessed by ultrasound or a procedure known as scintigraphy.
Hypothalamic pituitary axis (HPA). There is an intimate relationship between the thyroid, the adrenal glands and the sex hormones.76 If hypothyroidism is suspected, an evaluation of the adrenal glands as well as the sex hormones is suggested.