Tests to detect under- or over-activity of the thyroid gland are done quite often these days. Here's a primer about the commonest tests and what they can tell you.

Disorders of the thyroid gland are diagnosed more often nowadays, probably because testing has become more of a routine procedure. Two recent publications have provided information that we summarize here, to help you understand the value of blood tests for thyroid malfunction.

Simply put, the thyroid may be over-active or under-active. Whichever the direction, the disorder may produce obvious symptoms, or it may be unnoticed (i.e. what physicians call 'subclinical').

In USA adults, thyroid malfunction occurs roughly as follows:

The clinical signs mentioned earlier are shown in the following table:

Some abnormal lab test results can indicate that your thyroid is not working properly. Such findings include raised cholesterol, low serum sodium level, raised serum calcium, raised liver enzymes, and anemia.

It must be emphasized that mild under-activity of the gland can readily progress to clinically obvious under-activity, with an increased risk of high cholesterol levels, irregular heartbeat and osteoporosis.

Three blood tests are now commonly used to help diagnose malfunction of the thyroid. They are:

• Thyrotropin (thyroid stimulating hormone, or TSH, the pituitary hormone that controls the activity of the thyroid gland). The normal levels range from 1 to 6 mU/L.
• Free thyroxine (FT4, a thyroid hormone produced by the gland). Normal levels are 1 to 3 ng/dL.
• Free triiodothyronine (FT 3, another hormone produced by the thyroid) normal levels are 75 to 200 ng/dL.

Using these three tests, six types of malfunction can be recognized:

Because thyroid malfunction is quite common, and subclinical malfunction may require treatment to prevent other conditions developing, the American Thyroid Association recommends that thyrotropin is measured in adults every 5 years, starting at age 35. This simple screening test is every bit as cost-effectively as other widely accepted disease-detecting approaches, such as those for high blood pressure, high cholesterol, and breast cancer. If there are any symptoms or signs suggestive of thyroid malfunction, thyrotropin measurements should be done more frequently.

An abnormal thyrotropin result will probably necessitate additional tests, usually FT4 or FT3 in the first instance. More sophisticated tests may be ordered by your physician, or you may be advised to see a specialist. You should not be anxious about this, as thyroid diseases can almost always be managed quite satisfactorily.

Source

American Thyroid Association guidelines for detection of thyroid dysfunction. PW. Ladenson, PA. Singer, KB. Ain,  et al., Arch Int Med, 2000, vol. 160, pp. 1573--1575

Interpretation of thyroid function tests CM. Dayan, Lancet, 2001, vol. 357, pp. 619--624

Links

• Disorders of the thyroid gland
• Lessened thyroid activity and heart attacks
• Disease digests -- Hypothyroidism

The Thyroid Guide Your Thyroid: a Home Reference