TSH Testing
High TSH Means Low Thyroid Level---Low TSH Means High Thyroid Level

Thyroid experts recognize the TSH test as the test of choice when thyroid disease is suspected. The primary reason is that TSH begins to rise before thyroid hormones drop below the normal range. As such, it's a better indicator of early thyroid dysfunction. TSH is the hormone that comes from the pituitary gland and communicates with the thyroid gland, stimulating it to make and release thyroid hormones. Early in hypothyroidism, TSH goes up, trying to coax the thyroid to do its job.

If hypothyroidism is present, the thyroid won't be able to do that job properly. Meanwhile, TSH levels keep going up, continually trying to prod the thyroid back to normal production of thyroid hormones; the thyroid continues declining. After awhile, the thyroid hormones levels will drop below normal -- but by that time, hypothyroidism has already taken effect and is likely getting worse.

The TSH test is a good (but not perfect) test for thyroid disease. TSH values above the normal range indicate hypothyroidism, and the higher the number, the more severe the hypothyroidism. Low TSH values are usually caused by an overactive thyroid (hyperthyroidism).

Early hypothyroidism, when TSH is high, but thyroid hormone levels are normal is known as "subclinical hypothyroidism" or "mild thyroid failure." Many physicians are reluctant to treat this condition. Mild thyroid failure is increasingly being recognized as a real disease and is being treated. A recent article in the Annals of Internal Medicine found that mild thyroid failure more than doubles your risk of a heart attack.

Most of the time, if your TSH is mildly elevated, the usual diagnosis is mild thyroid failure. There are exceptions, however, to this rule. All of these exceptions are possible, but rare.

• Tumor. If the pituitary gland has a tumor, it may overproduce TSH on its own. Patients with this problem usually have symptoms of an overactive thyroid (hyperthyroidism), but those symptoms may overlap with those of an underactive thyroid.
  
• Medication. Anti-nausea medication (i.e.. Phenergan or Reglan) and psychiatric medications (i.e.. Haldol or Prolixin) can throw TSH tests out of whack.
  
• Timing. TSH should always be tested during the day. TSH levels often surge at night, and testing at this time can give an indication of TSH levels being high -- when, actually, they're normal.
  
• Immune problems. TSH antibodies can give false readings in the lab test, making a normal test appear high.

Link to American Thyroid Association website