What is Thyroid Testing?

The thyroid is a butterfly-shaped gland in your neck that makes hormones that regulate and control your metabolism. If your thyroid is overactive, you have hyperthyroidism, and if your thyroid is underactive, you have hypothyroidism. The pituitary gland makes a hormone called TSH, which stands for thyroid-stimulating hormone. It does this constantly. TSH is a hormone that helps control the activity of the thyroid gland. It does this by controlling how much thyroid hormone is produced to ensure the levels remain in a healthy range. If the pituitary gland detects low levels of thyroid hormone, it sends out more TSH to stimulate the thyroid to increase production. When levels are healthy again, it sends less. If it detects high levels, it reduces TSH, slowing the thyroid. When levels normalize, TSH is adjusted again.

The main thyroid hormones are free triiodothyronine (free T3) and thyroxine (free T4). These control how your body uses and stores energy. A high TSH means your thyroid is underactive. A low TSH means it’s overly active.

Hyperthyroidism and Hypothyroidism

Hyperthyroid: A chronically overactive thyroid produces too much free T3 and T4, even when TSH is low. This is seen in conditions like Graves' Disease.

Hypothyroid: A sluggish thyroid fails to produce enough free T3 and T4, despite high TSH. Hashimoto’s Disease is a common cause.

Autoimmune Thyroid Diseases

Hashimoto’s Disease: The immune system attacks an enzyme called thyroid peroxidase (TPO), essential for making thyroid hormones. This leads to high TSH, low free T4, and high anti-TPO antibodies.

Graves’ Disease: Antibodies called TSI stimulate the thyroid in place of TSH, leading to excess thyroid hormone. These patients have low TSH, high free T3 and T4, and high TSI levels.

Reverse T3: When there’s too much free T3, some of it may be inactivated and stored as reverse T3. It does not convert back to T4.

Why Are Thyroid Tests Ordered?

TSH is the best initial test. If it's abnormal, further testing is often done. A normal TSH usually rules out major thyroid issues.

How Should You Prepare for the Test?

• Do not take vitamins on the morning of your blood draw. If you are on thyroid medication, take your last dose 12–24 hours before testing. You can resume your medication immediately after your blood draw.

What Do the Results Mean?

If your TSH is high, your provider may suspect hypothyroidism and order a free T4 test. High TSH and low T4 = hypothyroidism. If both are high, other issues may be at play. If TSH is low, your provider may suspect hyperthyroidism. Low TSH and high T3/T4 confirm it. Some providers order reverse T3, but this is rarely used now. Providers also use thyroid testing to monitor treatment and adjust medications accordingly.

Questions to Ask Your Provider

Final Takeaway

The thyroid gland is key to healthy metabolism. TSH is the most useful initial test. Communicate your symptoms clearly and ask questions, especially if you’re on treatment or suspect thyroid issues.