Thyroid TSH Function Test

About this test

This test measures thyroid-stimulating hormone (TSH), the pituitary gland's chemical signal to the thyroid. TSH is the most sensitive first-line test for evaluating thyroid function, and is used to screen for hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid).

The thyroid regulates metabolism, energy, weight, heart rate, body temperature, and much more. Even mild dysfunction can cause noticeable symptoms. TSH is checked first because it rises before thyroid hormone levels fall (in hypothyroidism) and drops before they rise (in hyperthyroidism), making it the earliest and most reliable indicator of thyroid trouble.

Components

• TSH (Thyroid Stimulating Hormone, 3rd Generation): A high TSH means the pituitary is working hard to stimulate an underperforming thyroid — typically indicating hypothyroidism (which causes fatigue, weight gain, cold intolerance, and slow thinking). A low TSH means the pituitary has reduced its signal because there's too much thyroid hormone — typically indicating hyperthyroidism (which causes weight loss, rapid heart rate, anxiety, and heat intolerance).
• Reflex Free T4: If TSH is outside the normal range, the lab will automatically test Free T4 using the same blood sample at no additional draw required. Free T4 measures the active thyroid hormone circulating in your blood and helps confirm whether a true thyroid problem is present.

Who is this test for?

TSH screening is recommended by many guidelines for adults — particularly women over 60, pregnant women, and people with a family history of thyroid disease. It is also appropriate for anyone experiencing symptoms such as unexplained weight changes, fatigue, hair thinning, feeling unusually cold or hot, constipation, or mood shifts. People already on thyroid medication (such as levothyroxine) typically have TSH checked every six to twelve months.