Thyroid 101: TSH, T3 and T4

Your thyroid is a small gland with an outsized job: it sets your metabolic pace. When it drifts out of range, the symptoms — fatigue, weight changes, mood shifts, temperature sensitivity — are easy to blame on everything else. A thyroid panel cuts through the guesswork.

TSH — the thermostat

Thyroid-stimulating hormone comes from your pituitary, not your thyroid. It's the body's "request" for more thyroid hormone. Counterintuitively, a high TSH usually signals an underactive thyroid (the body is shouting for more), while a low TSH suggests an overactive one. TSH is the most sensitive first-line screen.

Free T4 and Free T3 — the actual hormones

T4 is the storage hormone; T3 is the active form your cells use. "Free" means the unbound, available fraction. Pairing these with TSH tells you not just that something is off, but where.

Common patterns

• High TSH, low Free T4: classic hypothyroidism (underactive).
• Low TSH, high Free T4/T3: hyperthyroidism (overactive).
• High TSH, normal Free T4: subclinical hypothyroidism — worth monitoring.

When antibodies matter

If your thyroid is off, autoimmune thyroid disease (Hashimoto's or Graves') is the most common cause. Thyroid antibody testing can confirm it and explain why levels fluctuate.

This article is for general education and is not medical advice. Consult a qualified healthcare provider about your results. Read about how we write about lab testing.