Blood tests that measure thyroxine and thyroid-stimulating hormone (TSH) can confirm the diagnosis. High levels of thyroxine and low or nonexistent amounts of TSH indicate an overactive thyroid. The amount of TSH is important because it’s the hormone that signals your thyroid gland to produce more thyroxine.

Specimen Required Serum separator tube or plasma separator tube. Also acceptable: Lavender (EDTA), pink (K2EDTA), or green (sodium or lithium heparin).

Key thyroid tests from Quest

Is Fasting Required for a Thyroid Test? Most doctors will suggest you do not fast before your thyroid function test. Research shows that fasting, especially early in the morning, may impact TSH levels. A fasting test typically results in higher TSH levels versus one done in the afternoon.

Coffee. The caffeine or coffee itself isn’t a problem, but many people drink coffee with cream or milk. The calcium in dairy products can interfere with thyroid hormone absorption. Wait an hour after taking your thyroid medication to have your cappuccino or latte.

Today’s TSH tests are very accurate and sensitive; they can help diagnose even the mildest cases of hypothyroidism. It’s important to understand that just because your TSH test comes back normal, it does not rule out the possibility of you being hypothyroid.

“Decreased absorption means that patients may not get the full dose of thyroid hormone that they are prescribed.” Although it makes sense that milk, which contains calcium, might interfere with levothyroxine absorption, no study has proved that it does until now, according to Chon.