Your thyroid panel includes TSH, Free T4, and sometimes Free T3 and antibodies. TSH is the most sensitive indicator — high TSH suggests hypothyroidism, low TSH suggests hyperthyroidism. Free T4 measures active thyroid hormone. Antibodies (TPO, TgAb) indicate autoimmune disease. Thyroglobulin monitors for thyroid cancer recurrence.
Understanding your thyroid labs can help you be a more informed participant in your care. This guide explains what each test measures, what normal and abnormal values mean, and how the results fit together.
TSH — The Master Control Test
TSH (thyroid-stimulating hormone) is produced by the pituitary gland and is the most sensitive indicator of thyroid function. Think of it as a thermostat:
- High TSH → pituitary is working hard to stimulate an underactive thyroid → suggests hypothyroidism
- Low TSH → pituitary is suppressed because thyroid hormone is too high → suggests hyperthyroidism
- Normal TSH range: approximately 0.4–4.0 mIU/L (varies slightly by lab)
🔑 TSH First: In most patients, TSH alone is the initial screening test for thyroid disease. Free T4 and T3 are added if TSH is abnormal or if specific situations require it (pituitary disease, T3 toxicosis, monitoring on treatment).
Free T4 — Active Thyroid Hormone
T4 (thyroxine) is the main hormone produced by the thyroid gland. "Free" T4 measures the unbound, biologically active fraction. Free T4 is ordered when:
- TSH is abnormal (to confirm and quantify the degree of dysfunction)
- Pituitary disease is suspected (TSH may be falsely normal)
- Monitoring thyroid replacement doses
Low Free T4 + High TSH = overt hypothyroidism
Normal Free T4 + High TSH = subclinical hypothyroidism (milder)
High Free T4 + Low TSH = overt hyperthyroidism
Free T3 — The Active Form
T3 (triiodothyronine) is the most biologically active thyroid hormone. Most T3 is made by converting T4 in peripheral tissues. Free T3 is ordered in specific situations:
- Suspected T3 toxicosis (hyperthyroidism with normal T4 but high T3)
- Monitoring patients on combination T4/T3 therapy (liothyronine)
- Evaluation of low T3 syndrome in chronic illness
Routine Free T3 testing in patients with hypothyroidism on levothyroxine alone is not recommended by major guidelines — though it is sometimes measured by patient request or in symptomatic patients despite normal TSH.
Thyroid Antibodies
| Antibody | What It Indicates | Clinical Use |
|---|---|---|
| TPO Ab (anti-thyroid peroxidase) | Autoimmune thyroid disease (Hashimoto's or Graves') | Confirms autoimmune cause; predicts future hypothyroidism |
| TgAb (anti-thyroglobulin) | Autoimmune thyroid disease | Also ordered in thyroid cancer surveillance (interferes with Tg assay) |
| TRAb / TSI (TSH receptor antibodies) | Graves' disease | Confirms Graves'; monitors treatment response; predicts remission |
Elevated TPO antibodies alone — without abnormal TSH or T4 — do not require treatment. They indicate a predisposition to thyroid disease and warrant periodic monitoring (annual TSH).
Thyroglobulin (Tg) — Cancer Surveillance
Thyroglobulin is produced exclusively by thyroid cells. After total thyroidectomy and radioactive iodine ablation for thyroid cancer, thyroglobulin should be undetectable. Rising Tg is a sensitive marker for cancer recurrence. It is not useful in patients with an intact thyroid gland as a screening test.
Subclinical Thyroid Disease
- Subclinical hypothyroidism: High TSH (typically 4–10 mIU/L) with normal Free T4. Treatment decisions depend on TSH level, symptoms, antibodies, age, and cardiovascular risk. TSH >10 mIU/L usually warrants treatment.
- Subclinical hyperthyroidism: Low TSH with normal Free T4 and T3. Risk-stratified approach — important in older patients due to atrial fibrillation and bone loss risk.
Key Takeaways
- TSH is the most sensitive thyroid test — high TSH = hypothyroidism, low TSH = hyperthyroidism
- Free T4 measures active thyroid hormone; 'free' is preferred over 'total'
- Free T3 is not routinely needed in most hypothyroid patients on levothyroxine
- TPO antibodies confirm autoimmune thyroid disease (Hashimoto's/Graves') — elevated antibodies alone don't require treatment
- Thyroglobulin (Tg) is used to monitor for thyroid cancer recurrence after surgery — not useful in routine thyroid screening
- Subclinical hypothyroidism (high TSH, normal T4) — treatment decision depends on TSH level, symptoms, and risk factors
- Share persistent symptoms with your endocrinologist even if TSH is 'normal' — clinical context always matters
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