The thyroid is a very complicated gland. It is responsible for producing hormones that direct our metabolism. As a result, symptoms of low thyroid function are a persistent feeling of coldness, lethargy and weight gain. People with an overactive thyroid will feel hot, somewhat jittery and will lose weight. Thyroid blood tests can help diagnose thyroid imbalances as well as the cause of the thyroid dysfunction.
Look at your TSH levels. TSH stands for thyroid stimulating hormone, and is normally between 0.5 and 5.5. TSH is responsible for the generation of the hormones that are in step 2.
Examine the levels of T3 and T4. These are the hormones that are actually responsible for the thyroid's effects. They are usually measured as "Free T3" and "Free T4," because the other tests can be easily thrown off by medications. T4 is normally at 0.8 to 1.8. T3 is four times as potent as T3 and is normally between 230 and 420.
Look at the relative levels of TSH and T3 and T4. If your TSH levels are low and your T3 and T4 levels are high, it means you are abnormally producing T3 and T4, which is causing your TSH levels to be suppressed. If your T3 and T4 levels are low along with low TSH levels, you aren't producing enough TSH. Finally, if your T3 and T4 levels are low and your TSH levels are high, you are just having trouble making T3 and T4.
Look at your ratio of T3 to T4. T3 is made from T4, and if your free T3 levels are low but your free T4 levels are high, you're having trouble with the conversion. Since T3 is more potent than T4, a problem with the conversion will give you overall low thyroid function.
Look for other tests that have been run. In some cases, other tests will also be run for compounds such as antibodies, free thyroxine index, thyroglobulin, and other blood tests. They may be ordered for certain suspected conditions, such as thyroid cancer, Hashimoto's thyroiditis and other diseases.