27/05/2026
Thyroid Function Test (TFT)
Definition
Thyroid Function Test (TFT) is a group of blood tests used to evaluate the function of the thyroid gland. These tests measure the levels of thyroid hormones and Thyroid Stimulating Hormone (TSH) in the blood.
The thyroid gland is located in the neck and produces hormones that regulate metabolism, growth, energy production, body temperature, and heart rate.
TFT helps diagnose thyroid disorders such as hypothyroidism, hyperthyroidism, thyroiditis, and other endocrine abnormalities.
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Principle
The thyroid gland mainly produces:
T3 (Triiodothyronine)
T4 (Thyroxine)
The pituitary gland secretes TSH (Thyroid Stimulating Hormone), which controls thyroid hormone production.
When thyroid hormone levels decrease, the pituitary gland releases more TSH. When thyroid hormone levels increase, TSH secretion decreases.
TFT measures these hormone levels using immunoassay techniques such as:
ELISA
CLIA
ECLIA
These tests help determine whether the thyroid gland is functioning normally.
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Objectives
1. To assess thyroid gland function.
2. To diagnose hypothyroidism and hyperthyroidism.
3. To monitor thyroid hormone therapy.
4. To detect congenital thyroid disorders.
5. To investigate metabolic and hormonal imbalance.
6. To evaluate pituitary-thyroid axis function.
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Specimen
Venous blood sample
Serum (preferred)
Plasma may also be used
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Components of TFT
1. TSH (Thyroid Stimulating Hormone)
Produced by the pituitary gland and regulates thyroid hormone production.
Normal Range
0.4 – 4.0 mIU/L
Increased TSH
Seen in:
Hypothyroidism
Hashimoto thyroiditis
Decreased TSH
Seen in:
Hyperthyroidism
Graves’ disease
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2. T3 (Triiodothyronine)
Active thyroid hormone controlling metabolism.
Normal Range
80 – 200 ng/dL
Increased T3
Hyperthyroidism
Decreased T3
Hypothyroidism
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3. T4 (Thyroxine)
Main hormone produced by thyroid gland.
Normal Range
4.5 – 12.0 µg/dL
Increased T4
Hyperthyroidism
Decreased T4
Hypothyroidism
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4. Free T3 (FT3)
Unbound active form of T3.
Normal Range
2.0 – 4.4 pg/mL
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5. Free T4 (FT4)
Unbound active form of T4.
Normal Range
0.8 – 2.0 ng/dL
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Materials Required
Syringe or vacutainer
Plain blood collection tube
Centrifuge
Immunoassay analyzer
TFT reagent kit
Pipette
Gloves and cotton
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Procedure
1. Collect venous blood sample using aseptic technique.
2. Transfer blood into plain tube.
3. Allow blood to clot.
4. Centrifuge sample to separate serum.
5. Transfer serum into labeled tube.
6. Load serum into analyzer.
7. Add TFT-specific reagents.
8. Analyzer measures hormone levels.
9. Record and interpret results.
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Observation
Increased T3/T4 with decreased TSH indicates hyperthyroidism.
Decreased T3/T4 with increased TSH indicates hypothyroidism.
Normal hormone levels indicate normal thyroid function.
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Clinical Significance
Hypothyroidism
Occurs due to reduced thyroid hormone production.
Causes
Hashimoto thyroiditis
Iodine deficiency
Thyroid surgery
Pituitary disorders
Symptoms
Fatigue
Weight gain
Cold intolerance
Dry skin
Constipation
Slow heartbeat
Depression
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Hyperthyroidism
Occurs due to excessive thyroid hormone production.
Causes
Graves’ disease
Toxic multinodular goiter
Thyroid adenoma
Symptoms
Weight loss
Heat intolerance
Nervousness
Tremors
Sweating
Palpitations
Increased appetite
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Result Interpretation
TFT Findings Interpretation
High TSH + Low T3/T4 Hypothyroidism
Low TSH + High T3/T4 Hyperthyroidism
Normal TSH, T3, T4 Normal thyroid function
High TSH + Normal T4 Subclinical hypothyroidism
Low TSH + Normal T4 Subclinical hyperthyroidism
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Methods Used in TFT
1. ELISA (Enzyme Linked Immunosorbent Assay)
2. CLIA (Chemiluminescent Immunoassay)
3. ECLIA (Electrochemiluminescence Immunoassay)
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Precautions
1. Use properly labeled tubes.
2. Avoid hemolysis of blood sample.
3. Separate serum promptly.
4. Follow analyzer instructions carefully.
5. Store reagents at recommended temperature.
6. Avoid contamination during testing.
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Advantages
Early detection of thyroid disorders
Accurate hormonal evaluation
Useful for treatment monitoring
Helps assess metabolic disorders
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Limitations
Certain drugs may affect results.
Pregnancy can alter thyroid hormone levels.
Severe illness may interfere with TFT values.
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Conclusion
Thyroid Function Test (TFT) is an important hormonal investigation used to assess thyroid gland activity and diagnose thyroid disorders. It plays a major role in evaluating metabolism, endocrine balance, and treatment response in patients with thyroid disease.
