Thyroid disorders are among the most common endocrine conditions globally, yet they are often under-diagnosed because symptoms can be vague and gradual. A thyroid function test (TFT) panel — typically TSH, Free T4, and sometimes Free T3 — is the primary tool for diagnosing and monitoring thyroid disease.
TSH: The Master Regulator
Thyroid Stimulating Hormone (TSH), secreted by the pituitary gland, is the single most sensitive test for thyroid dysfunction. It operates on a feedback loop: when thyroid hormone levels fall, the pituitary releases more TSH to stimulate the thyroid gland. The normal adult range is 0.4–4.0 mIU/L.
- High TSH (> 4.0) → hypothyroidism (underactive thyroid)
- Low TSH (< 0.4) → hyperthyroidism (overactive thyroid)
- TSH is normal → thyroid function is almost certainly normal
- During pregnancy (first trimester): upper limit is 2.5 mIU/L
TSH is the best single screening test for thyroid disease. However, it alone cannot tell you the type or severity — that requires Free T4.
Free T4: Confirming the Diagnosis
Free Thyroxine (Free T4) is the active, unbound form of the main thyroid hormone. Normal adult range: 0.8–1.8 ng/dL (10.3–23.2 pmol/L). Free T4 confirms the picture suggested by TSH. High TSH + low Free T4 = overt hypothyroidism. Low TSH + high Free T4 = overt hyperthyroidism. High TSH + normal Free T4 = subclinical hypothyroidism.
Free T3: The Active Hormone
Free T3 (Triiodothyronine) is the most biologically active thyroid hormone. It is mainly ordered to diagnose T3 toxicosis (hyperthyroidism with normal T4) and to monitor patients on thyroid replacement therapy. Normal adult range: 2.3–4.2 pg/mL (3.5–6.5 pmol/L).
Anti-TPO: Autoimmune Thyroid Disease
Anti-thyroid peroxidase (Anti-TPO) antibodies attack the thyroid gland. Elevated Anti-TPO (> 34 IU/mL) indicates autoimmune thyroid disease — most commonly Hashimoto's thyroiditis (hypothyroidism) or Graves' disease (hyperthyroidism). It is important for prognosis: high Anti-TPO in subclinical hypothyroidism significantly increases the risk of progression to overt disease.
A mildly elevated TSH (4–10 mIU/L) with positive Anti-TPO in a symptomatic patient often warrants treatment — discuss with your endocrinologist.
Common Patterns at a Glance
Here is how to read the most common thyroid test combinations:
- Normal TSH, normal FT4 → Euthyroid (thyroid is normal)
- High TSH, low FT4 → Overt hypothyroidism
- High TSH, normal FT4 → Subclinical hypothyroidism
- Low TSH, high FT4/FT3 → Overt hyperthyroidism / Graves' disease
- Low TSH, normal FT4/FT3 → Subclinical hyperthyroidism
- Low TSH, low FT4 → Central/pituitary hypothyroidism (rare)
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Dr. Naeem Mahmood Ashraf
PhD Biochemistry & Biotechnology
University of Punjab, Lahore
Dr. Naeem Mahmood Ashraf is a distinguished biochemist and biotechnologist at the University of Punjab, Lahore, Pakistan. With a PhD in Biochemistry & Biotechnology and over 45 peer-reviewed publications (h-index: 10), Dr. Ashraf brings deep expertise in clinical biochemistry, genomics, and computational biology to LabSense AI. His research bridges laboratory science and patient care, ensuring all interpretations follow WHO, IFCC, and AACC international standards.
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