When it comes to diagnosing hypothyroidism, it's often important to perform several tests and to consider all the results before making a formal diagnosis. Although significantly elevated levels of TSH generally do indicate an underactive thyroid, a single test is sometimes not enough to determine whether you have hypothyroidism.

Instead, your doctor needs to consider all your test results as well as the physical exam of your thyroid and your self-reported symptoms. In some cases, all the tests may come back normal, despite the presence of numerous symptoms of hypothyroidism. If that's the case, you may need to be retested in six months.

But if the tests do detect abnormalities, you may be diagnosed with some form of hypothyroidism. Here are some possible diagnoses, based on your lab results:

• Mild (Subclinical) Hypothyroidism: If you are mildly hypothyroid, you may have a normal free T3 test and a normal free T4 test. But your TSH levels may be high. See below for more discussion on this topic.

• Hypothyroidism: If you have hypothyroidism as a result of a deficiency in thyroid hormone, you will probably have a normal or low free T3 test, a low free T4 test, and elevated levels of TSH.

• Hashimoto's Disease: If you test positive for anti-TPO antibodies in addition to a normal or low free T3 test, low free T4 test, and high levels of TSH, your hypothyroidism may be caused by Hashimoto's thyroiditis. You may also have an enlarged thyroid gland.

• Pituitary (Central) Hypothyroidism: In rare cases, disease can affect your pituitary gland, making it unable to produce enough TSH to stimulate your thyroid gland to produce enough thyroid hormone. As a result, your TSH levels will be low or low-normal, and your free T3 and free T4 levels may be low or normal. You may also have symptoms of pituitary dysfunction, including menstrual irregularities and low libido.