Thyroid function tests (TFTs) are essential for diagnosing and managing thyroid disorders. These include measurement of TSH (Thyroid Stimulating Hormone), FT3 (Free Triiodothyronine), FT4 (Free Thyroxine), and Anti-TPO antibodies. Interpretation of these tests helps clinicians distinguish between hypothyroidism, hyperthyroidism, and autoimmune thyroid conditions like Hashimoto’s thyroiditis and Graves’ disease.
🧪 Key Thyroid Function Tests Explained
- TSH: Produced by the pituitary gland, it regulates thyroid hormone production. It is the most sensitive test for primary thyroid dysfunction.
- FT3: The active form of thyroid hormone, affects metabolism and energy levels.
- FT4: A precursor to FT3, also important in metabolic regulation.
- Anti-TPO: Autoantibodies against thyroid peroxidase, indicating autoimmune thyroiditis.
📊 Reference Ranges
| Test | Normal Range |
|---|---|
| TSH | 0.4 – 4.0 µIU/mL |
| FT4 | 0.8 – 2.0 ng/dL |
| FT3 | 2.3 – 4.1 pg/mL |
| Anti-TPO Antibodies | < 35 IU/mL |
🔍 Interpretation Patterns
| Condition | TSH | FT4 | FT3 | Anti-TPO |
|---|---|---|---|---|
| Primary Hypothyroidism | ↑ | ↓ | ↓ | ↑ (if autoimmune) |
| Subclinical Hypothyroidism | ↑ | Normal | Normal | ↑ |
| Primary Hyperthyroidism | ↓ | ↑ | ↑ | ↑ (Graves’) |
| Subclinical Hyperthyroidism | ↓ | Normal | Normal | ± |
| Central Hypothyroidism | ↓ | ↓ | ↓ | Normal |
💡 Clinical Notes
Pregnancy and TSH
Trimester-specific reference ranges are recommended. For example, in the first trimester, TSH should be below 2.5 µIU/mL. FT4 should also be interpreted carefully due to increased binding proteins.
🧾 When Should Thyroid Function Tests Be Ordered?
- Symptoms of fatigue, weight changes, or cold/heat intolerance
- Irregular menstrual cycles or infertility
- Neck swelling or goiter
- Monitoring of thyroid hormone therapy
- Autoimmune disease or family history of thyroid disorders
🔗 Related Articles
❓ Frequently Asked Questions
1. Is TSH always the best test?
TSH is the most sensitive indicator for primary thyroid disorders, but should be combined with FT4/FT3 in special cases (e.g., central hypothyroidism or pregnancy).
2. What does a high Anti-TPO mean in euthyroid patients?
They may be at risk of future hypothyroidism. Regular follow-up is advised.
3. Can stress affect TSH levels?
Yes, severe illness or stress can transiently suppress TSH (sick euthyroid syndrome).
✅ Conclusion
Understanding thyroid function tests is essential in diagnosing thyroid disorders and guiding appropriate therapy. TSH, FT3, FT4, and Anti-TPO together provide a complete picture of thyroid health.
Tags: Thyroid Function Tests, TSH, FT3, FT4, Anti-TPO, Hypothyroidism, Hyperthyroidism, Hashimoto’s, Graves’, Autoimmune thyroid disease, Clinical Biochemistry
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