Thyroid Lab Tests Explained

Research published in the British Medical Journal (BMJ 2000;320:1332-1334) pointed out problems with lab testing for hypothyroidism. TSH, the main test used to screen for thyroid disease, is inadequate, and many cases of hypothyroidism are missed—especially when it is the only test performed.

Do not rely solely on a TSH test. Ask your doctor to run a complete thyroid panel. Even then, thyroid testing sometimes requires reading between the lines.

Thyroid Tests

TSH

TSH is the most commonly used screening test for thyroid problems.

In patients with symptoms of hypothyroidism and a low basal body temperature:

• TSH above about 2.0 may suggest the thyroid gland itself is underperforming.
• Many patients with TSH around 3.0 or higher report symptoms even though they are often told their thyroid tests are “normal.”

If TSH is below about 2.0 and symptoms are present, the issue may sometimes involve the pituitary gland, which controls thyroid hormone production.

T4

T4 is the primary hormone produced by the thyroid.

If T4 levels are below the midpoint of the laboratory range and hypothyroid symptoms are present, inadequate intake of nutrients such as iodine or tyrosine may play a role.

T3

T3 is the active form of thyroid hormone.

The body must convert T4 into T3 for thyroid hormone to work properly. If T4 levels are adequate but T3 levels are below the midpoint of the laboratory range, the body may not be converting thyroid hormone efficiently.

Stress, hormonal imbalance, or nutrient deficiencies can interfere with this conversion.

Reverse T3

Another test, reverse T3 (RT3), measures an inactive form of thyroid hormone. If RT3 is elevated, it may look like your are making plenty of T3 but are not, unless you test for it.

Elevated reverse T3 may interfere with thyroid hormone activity and can sometimes make thyroid tests appear normal in patients who still have symptoms of hypothyroidism.