Thyroid (TSH) Test

What it is

The thyroid does not decide on its own how much hormone to make. The pituitary gland reads the level in the blood and sends TSH to push the thyroid harder or ease off. Measuring TSH therefore measures the body's own verdict on whether there is enough thyroid hormone circulating, which is why it is the usual opening test rather than a direct measurement of thyroid hormone. The relationship runs backwards from what people expect: a high TSH generally means an underactive thyroid, because the pituitary is pushing harder, and a low TSH generally means an overactive one.

Why it is done

• To investigate symptoms that could be thyroid related, such as unexplained weight change, fatigue, temperature intolerance or a changed heart rate
• To check the dose in someone taking thyroid hormone replacement, which is probably the most common reason this test is repeated
• To follow up on a thyroid nodule, an enlarged thyroid or treatment for an overactive thyroid
• As part of newborn screening, where an untreated congenital thyroid problem has consequences that testing avoids

What to expect

A blood draw from the arm, no fasting required. Some medications and supplements affect the result or the timing of the draw, so the ordering clinician may give specific instructions about when to take a morning dose. Results generally return within a day or two. A single borderline TSH is often repeated before anything is concluded, since the value moves with illness, time of day and pregnancy.

Questions worth asking

• Is this ordered as a reflex test? A TSH set up to reflex automatically to free T4 when it falls outside a range will produce a second lab charge without another draw.
• Was a thyroid panel ordered instead of a single TSH, and was the extra breadth intended?
• If TSH is being rechecked every few weeks while a dose is adjusted, how many of those checks will the plan cover?
• Is the draw being done at a hospital outpatient lab rather than a freestanding one? The facility setting can change the price of an identical test.