Understanding your thyroid test (TSH)

TSH (thyroid-stimulating hormone) is a message from your pituitary gland. It tells your thyroid how hard to work. It is the most common first blood test of thyroid function. As a rule of thumb, a high TSH often points to an underactive thyroid. A low TSH points to an overactive one. But the number is always read alongside your symptoms and other tests.

What does TSH measure?

TSH is made by the pituitary gland to control the thyroid. When thyroid hormone levels fall, the pituitary releases more TSH to push the thyroid. When they rise, it releases less. That feedback loop is why TSH is such a sensitive first check [American Thyroid Association, 2014].

What is a normal TSH range?

Many labs use a reference range of roughly 0.4 to 4.0 mIU/L. The exact range varies by lab, test method, age, and pregnancy status [American Thyroid Association, 2014]. Always read the range printed on your own report.

What does a high TSH mean?

A high TSH suggests the pituitary is working harder to push a thyroid that may be underactive. Doctors call this hypothyroidism. On its own it is a signal to discuss, not a diagnosis. Your doctor may look at free T4 and your symptoms before deciding anything [American Thyroid Association, 2014].

What does a low TSH mean?

A low TSH can suggest an overactive thyroid, called hyperthyroidism. Here thyroid hormone is high, so the pituitary dials back. Again, it is read with other tests and how you feel [American Thyroid Association, 2014].

When should TSH be rechecked?

TSH can shift with illness, medication, and time of day. So a single borderline value is often repeated before any conclusions. Ask your doctor when a re-check makes sense for you.

Frequently asked questions

Does a slightly high TSH mean I need medication?

Not necessarily. A mildly raised TSH is often rechecked. It is weighed alongside symptoms and other thyroid tests before any decision [American Thyroid Association, 2014].

Can anything temporarily change my TSH?

Yes. Recent illness, some medicines, and even the time of the blood draw can nudge TSH. This is one reason results are read in context.

Do I need to fast for a TSH test?

Fasting is not usually required for TSH. But follow your lab's specific instructions, especially if other tests are done at the same time.