The thyroid gland — a small butterfly-shaped organ at the base of your neck — controls your body's metabolism, energy production, heart rate, body temperature, and dozens of other critical functions. Thyroid Function Tests (TFT), also called thyroid panel or thyroid profile, are blood tests that measure how well your thyroid gland is working. Understanding your TFT results can explain symptoms ranging from unexplained fatigue and weight gain to anxiety and heart palpitations.
What Tests Are Included in a TFT Panel?
A standard thyroid function test panel typically includes the following markers. Your doctor may order all of them or a subset depending on your clinical situation.
- TSH (Thyroid Stimulating Hormone) — the most sensitive marker for thyroid function
- Free T4 (Free Thyroxine / FT4) — the main hormone produced by the thyroid gland
- Free T3 (Free Triiodothyronine / FT3) — the biologically active form of thyroid hormone
- Total T4 and Total T3 — less commonly ordered; measures both free and protein-bound hormone
- Anti-TPO antibodies — ordered to diagnose autoimmune thyroid disease (Hashimoto's or Graves' disease)
- Anti-thyroglobulin antibodies — additional autoimmune marker
- Reverse T3 (rT3) — ordered in complex cases to assess conversion problems
TSH (Thyroid Stimulating Hormone) — The Master Marker
TSH is produced by the pituitary gland in the brain and controls how much hormone the thyroid produces. It is the single most important test for thyroid function and is almost always the first test ordered. The normal TSH range is 0.4–4.0 mIU/L in most laboratories, though some use 0.5–4.5 mIU/L.
- TSH < 0.4 mIU/L: Suggests hyperthyroidism — thyroid is overactive, pituitary reduces stimulation
- TSH 0.4–4.0 mIU/L: Normal thyroid function in most people
- TSH 4.0–10.0 mIU/L: Subclinical hypothyroidism — mildly underactive thyroid
- TSH > 10.0 mIU/L: Overt hypothyroidism — thyroid is significantly underactive
- TSH > 100 mIU/L: Severe hypothyroidism — requires urgent medical attention
TSH has an inverse relationship with thyroid function. A HIGH TSH means the pituitary is working harder to stimulate an underperforming thyroid. A LOW TSH means the pituitary is suppressing stimulation because the thyroid is already overproducing.
Free T4 (FT4) — What the Thyroid Actually Produces
Free T4 is the primary hormone secreted by the thyroid gland. "Free" means it is unbound to proteins and biologically available. It is converted to the active T3 in peripheral tissues. Normal free T4 ranges are typically 0.8–1.8 ng/dL (10–23 pmol/L).
- High FT4 with low TSH: Confirms hyperthyroidism (Graves' disease, toxic nodule)
- Low FT4 with high TSH: Confirms primary hypothyroidism (Hashimoto's thyroiditis, iodine deficiency)
- Low FT4 with low TSH: Suggests central hypothyroidism — pituitary or hypothalamus problem
- Normal FT4 with high TSH: Subclinical hypothyroidism — treatment decision depends on symptoms and antibodies
Free T3 (FT3) — The Active Thyroid Hormone
Free T3 is the biologically active form of thyroid hormone — the one that actually enters your cells and drives metabolism. T4 is converted to T3 in the liver, kidneys, and other tissues. Normal free T3 is 2.3–4.2 pg/mL (3.5–6.5 pmol/L). T3 is particularly useful when T4 is normal but symptoms persist.
- Low FT3 with normal FT4 and TSH: Poor T4 to T3 conversion — common in chronic stress, selenium deficiency, low-calorie diets
- High FT3 with suppressed TSH: T3 toxicosis — rare form of hyperthyroidism
- Low FT3 in critically ill patients: "Sick euthyroid syndrome" — not true hypothyroidism
Many patients with Hashimoto's thyroiditis have normal TSH and FT4 but low FT3, which explains persistent symptoms despite being told their thyroid tests are "normal." Always ask for FT3 if you have ongoing symptoms.
Hypothyroidism — Reading the Low Thyroid Pattern
Hypothyroidism (underactive thyroid) is the most common thyroid disorder worldwide, affecting approximately 5% of the population — more common in women and those over 60. The classic TFT pattern for hypothyroidism is a HIGH TSH combined with a LOW free T4.
- Symptoms: fatigue, weight gain, cold intolerance, constipation, dry skin, hair loss, brain fog, depression
- Primary hypothyroidism (most common): High TSH + Low FT4 — thyroid gland itself is failing
- Subclinical hypothyroidism: High TSH + Normal FT4 — early stage, may or may not have symptoms
- Hashimoto's thyroiditis: Autoimmune cause — positive Anti-TPO antibodies confirm this
- Treatment: Levothyroxine (synthetic T4) — dose titrated to bring TSH to 1–2.5 mIU/L
Hyperthyroidism — Reading the High Thyroid Pattern
Hyperthyroidism (overactive thyroid) means the thyroid is producing too much hormone, speeding up body metabolism. The classic TFT pattern is a LOW TSH combined with HIGH free T4 and/or free T3.
- Symptoms: weight loss despite increased appetite, rapid heart rate, tremors, anxiety, insomnia, heat intolerance, excessive sweating
- Graves' disease: Most common cause — autoimmune condition, TSH receptor antibodies (TRAb) positive
- Toxic nodular goitre: One or more thyroid nodules producing hormone autonomously
- Thyroiditis: Temporary inflammation releasing stored hormone — TSH suppressed transiently
- Treatment options: Anti-thyroid drugs (methimazole, propylthiouracil), radioactive iodine, or surgery
Untreated hyperthyroidism can lead to atrial fibrillation, osteoporosis, and thyroid storm — a life-threatening emergency. A low TSH should never be ignored.
TFT Interpretation Quick Reference Table
Use this reference to quickly interpret your thyroid test pattern:
- TSH High + FT4 Low = Primary Hypothyroidism
- TSH High + FT4 Normal = Subclinical Hypothyroidism
- TSH Low + FT4 High = Primary Hyperthyroidism
- TSH Low + FT4 Normal = Subclinical Hyperthyroidism or T3 toxicosis
- TSH Low + FT4 Low = Central Hypothyroidism (pituitary disorder)
- TSH Normal + FT4 Normal + FT3 Low = Poor T4→T3 conversion
- All normal = Euthyroid (normal thyroid function)
Factors That Affect TFT Results
Several factors can alter thyroid function test results without any actual thyroid disease being present. Always inform your doctor about these when discussing your results.
- Biotin supplements: High-dose biotin (vitamin B7) causes falsely low TSH and falsely high T4 — stop biotin 48–72 hours before the test
- Time of day: TSH is highest in the early morning and lowest in the afternoon — always test at the same time for consistency
- Pregnancy: Normal TSH is lower in first trimester (0.1–2.5 mIU/L); different ranges apply throughout pregnancy
- Medications: Lithium, amiodarone, steroids, metformin, and many others affect thyroid markers
- Acute illness: "Sick euthyroid syndrome" — severe illness suppresses TSH and T3 transiently
- Iodine deficiency or excess: Both can cause thyroid dysfunction
Get Your TFT Results Explained by AI
If you have received a thyroid function test report and are unsure what the numbers mean for your specific situation, our free AI Lab Result Interpreter at LabSense AI can analyse your complete TFT panel — TSH, FT4, FT3, and antibody results — and explain each value in plain language, including whether your results suggest hypothyroidism, hyperthyroidism, or normal thyroid function. Upload your report at labsense-ai.com/interpreter for a personalised interpretation within seconds.
Frequently Asked Questions
What is the most important test in a TFT panel?▼
TSH (Thyroid Stimulating Hormone) is the single most sensitive and important marker in a thyroid function test. It is the first test ordered and reflects overall thyroid gland performance. A normal TSH generally rules out significant thyroid dysfunction in most patients.
What does a TSH of 6.5 mean?▼
A TSH of 6.5 mIU/L is above the normal upper limit of 4.0–4.5 mIU/L, indicating subclinical or mild hypothyroidism. Whether treatment with levothyroxine is recommended depends on your free T4 level, symptoms, Anti-TPO antibody status, and risk factors. Your doctor will assess the clinical picture before deciding.
Can stress affect TFT results?▼
Yes. Chronic psychological stress elevates cortisol, which suppresses TSH and reduces T4-to-T3 conversion, leading to low free T3. Acute severe physical illness can cause "sick euthyroid syndrome" where TSH drops temporarily. This is why thyroid tests should ideally be done when you are in a stable health state.
What is the difference between T3 and T4?▼
T4 (thyroxine) is the main hormone produced by the thyroid gland and acts as a storage/transport form. It is converted to T3 (triiodothyronine) in body tissues — T3 is the biologically active form that actually controls metabolism at the cellular level. T4 has 4 iodine atoms; T3 has 3. T3 is approximately 4 times more potent than T4.
Is a low TSH always a sign of hyperthyroidism?▼
Not always. A low TSH can also occur in the first trimester of pregnancy (normal), with high-dose biotin supplements (false low), after thyroid medication overtreatment, or in sick euthyroid syndrome during serious illness. Context is critical — low TSH should always be evaluated alongside free T4 and T3 before a diagnosis is made.
How often should I get my thyroid tested?▼
For people with known hypothyroidism on stable levothyroxine therapy, annual TSH testing is typically sufficient. If dosage is being adjusted, testing every 6–8 weeks is recommended until stable. For people with risk factors (family history, autoimmune disease, previous thyroid disease), screening every 3–5 years is reasonable. Pregnant women should be tested at the first antenatal visit and each trimester.
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Medical Advisory
Expert oversight & content review
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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