Understanding Your Thyroid Test

The thyroid is a small, butterfly-shaped gland in your neck that produces hormones controlling your metabolism, energy, heart rate and body temperature. When it produces too much or too little hormone, it can affect nearly every system in your body. A thyroid function test is a simple blood test that helps your doctor check how well your thyroid is working.

What does a thyroid test measure?

A typical thyroid profile measures three key markers:

• TSH (Thyroid Stimulating Hormone) — produced by the pituitary gland; it signals the thyroid to make hormones. TSH is the most sensitive early indicator of thyroid problems.
• T4 (Thyroxine) — the main hormone the thyroid releases, mostly in an inactive storage form.
• T3 (Triiodothyronine) — the active hormone that drives your metabolism.

Typical reference ranges

Ranges vary slightly by laboratory, method and, for T4/T3, whether "total" or "free" is measured. General adult reference ranges are:

What do abnormal results mean?

Hypothyroidism (underactive thyroid)

Usually shows a high TSH with low T4. Symptoms include tiredness, weight gain, cold intolerance, dry skin, constipation and low mood.

Hyperthyroidism (overactive thyroid)

Usually shows a low TSH with high T3/T4. Symptoms include weight loss, rapid heartbeat, anxiety, tremor, sweating and difficulty sleeping.

Who should get tested?

• Anyone with symptoms of an under- or over-active thyroid;
• Women planning pregnancy or who are pregnant;
• People with a family history of thyroid disease;
• Those on thyroid medication (for monitoring).

How to prepare

No fasting is generally required for a thyroid test. If you take thyroid medication, ask your doctor whether to take it before or after the blood draw, as timing can affect results. Mention any supplements (especially biotin) as they can interfere with some assays.