Some people feel much better when taking thyroid hormones, even though some thyroid tests might indicate that they do not need it. For sub-clinical cases, a short-term thyroid trial is appropriate. The best lab tests to consider if low thyroid function is suspected are TSH (Thyroid Stimulating Hormone), Free T-4, and Free T-3. Your doctor may or may not require lab testing before giving you thyroid hormone on a clinical trial basis, when symptoms indicate. If your doctor refuses to give you a clinical trial and does not provide a convincing reason, you may wish to get a second opinion.

Oral temperatures during the day are a reasonable method of monitoring your thyroid status and the effectiveness of any thyroid medication. Since the thyroid regulates the body's metabolic rate, greater thyroid activity usually results in a higher body temperature. Your body temperature should be greater than 98.2F (36.8C).

The American Association of Clinical Endocrinologists officially stated in 2001, that a "TSH level between 3.0 and 5.0 uU/ml ... should be considered suspect since it may signal a case of evolving thyroid underactivity." This is a dramatic reversal, as they have traditionally held that patients should never be treated with thyroid hormone unless the tests were clearly abnormal, above 5.5. This supports the belief that many patients have been misdiagnosed and undertreated in the past based on the lab tests and the arbitrary normal ranges. Testing should also be done when hyperthyroidism is suspected. A low TSH is suggestive, but if borderline, a free T-3 can be done.

Thyroid lab testing can be done while you are on your thyroid medication. Some doctors recommend taking your daily thyroid dose after any thyroid lab testing is done, not before.

Free T3 testing may be needed to determine if hypothyroidism (or hyperthyroidism) is an issue and if it is being effectively treated. T4 is converted to T3 in the body, and T3 is much more metabolically active than T4. In normal thyroid function, as the concentrations of the carrier proteins change, the total triiodothyronine (T3) level also changes, leaving the free T3 concentration relatively the same (In an abnormally functioning thyroid, this is not necessarily so). Measurements of free T3 concentrations, therefore, correlate more reliably with your clinical status than either total T3 or T4 levels. For those who are taking T4 as their only thyroid medication, it is sometimes helpful to determine what their free T3 level is, to make sure that good conversion of T4 to T3 is taking place.

In cases where the TSH is elevated but hypothyroid symptoms are not present, thyroid enlargement or tenderness suggests autoimmunity. Antithyroid antibodies such as antithyroglobulin and antiperoxidase may be atacking the thyroid gland. These autoantibodies can be found on blood testing. If the TSH is as high as 10 or so, but there are no autoantibodies or symptoms, some doctors will not treat, but watch the condition to see if there is any progression.