Diagnosis

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Thyroid hormone (TH), which is stored and produced by the thyroid gland, actually consists of two hormones: thyroxine, known as T4, and tri-iodothyronine, known as T3. The production of T4 and T3 is controlled by thyroid stimulating hormone (TSH). The pituitary gland produces TSH, controlling the production of TH by the thyroid gland. The pituitary gland acts like a sensor on a thermostat; if it senses too little TH in your blood, it releases TSH to tell your thyroid to produce more. Likewise, if your pituitary senses too much TH in the blood, it decreases production of TSH.

The best way to determine if your body is making too much or too little TH is by measuring blood levels of TSH. If the TSH level is abnormal, your health care professional may also want to test your blood for T3 and T4 levels. These blood tests provide an accurate picture of how the thyroid is functioning.

Hypothyroidism is diagnosed if TH levels are low to normal and TSH levels are high. To rule out Hashimoto's thyroiditis as the cause, your health care professional may check your blood for antithyroid antibodies.

Hyperthyroidism is suspected if TH levels are high and TSH levels are low. To determine if Graves' disease is the cause, your health care professional can check your blood for thyroid stimulating antibodies or give you a radioactive iodine uptake test.

Iodine is essential for the production of TH, so the thyroid absorbs it from the blood. During a radioactive iodine uptake test, you swallow a small amount of slightly radioactive iodine. The thyroid absorbs and metabolizes this radioactive iodine within 24 hours. Special equipment is then used to measure the amount of radioactivity in the thyroid gland, and you usually have to return within six and 24 hours to have the radioactivity measured (although some labs only measure after 24 hours). If you have Graves' disease, the amount of radioactivity in the thyroid is high. If you have other forms of hyperthyroidism, such as an inflammation of the thyroid known as a thyroiditis, the radioactivity taken up by the thyroid will be low.

Thyroid Nodules

There are four ways to diagnose a thyroid nodule:

• You or your health care professional feels a growth while manually examining your throat, even though you have no symptoms.

• You have trouble swallowing and, upon evaluation, your health care professional identifies a nodule.

• You have symptoms of hypothyroidism or hyperthyroidism.

• You have an imaging study of your neck performed for an unrelated reason (i.e., CT scan, MRI, carotid ultrasound) that discovers the nodule.

All nodules should be evaluated by a specialist, such as an endocrinologist or an internal medicine specialist, to determine if the nodule is caused by a thyroid cancer. In addition to ordering blood tests described earlier, your doctor will examine the structure of the thyroid gland using one or more of the following tests:

• Biopsy. In this test, called a fine needle aspirate, a very thin needle is inserted into the gland, and several samples of tissue are sucked out (aspirated). The samples are then analyzed under a microscope. This is the best test to determine if a thyroid cancer is present. A biopsy is usually performed if the nodule is larger than 1.5 cm and occasionally with smaller nodules depending on your risk factors and how concerned you and your doctor are.

• Thyroid scan. As with a radioactive iodine uptake test described earlier, in a thyroid scan you swallow a radioactive chemical (usually iodine or technetium). A special camera is then used to view the size, shape and function of the gland based on the amount of radioactive material absorbed. This helps determine whether the nodule is "hot" (usually benign but overactive) or "cold" (inactive and either benign or malignant).

• Ultrasound. An ultrasound test uses sound waves to determine whether a nodule is filled with fluid or solid. If the nodule is filled with fluid, it suggests a thyroid cyst. A solid nodule doesn't necessarily mean cancer, but it may mean that further testing is required. This test can also find other nodules that can't be felt with a manual examination. It is often used to guide biopsies of nodules. A thyroid ultrasound is also the best test to determine the size of the nodules and to follow any growth over time.