🧠 Thyroid Hormone Panel – Complete Explanation

1. Free T3 (Triiodothyronine – Unbound Form)

• ✅ Fasting: Recommended (for standardization)
• 📍 Produced By: Thyroid and peripheral conversion from T4
• 📊 Measures: Active, unbound T3 available to tissues
• 📈 High: Hyperthyroidism (T3 toxicosis), Graves’ disease
• 📉 Low: Hypothyroidism, chronic illness (NTIS)
• ✅ Normal Range: 2.3 – 4.2 pg/mL
• 🩺 Most active thyroid hormone at the tissue level

2. Free T4 (Thyroxine – Unbound Form)

• ✅ Fasting: Yes
• 📍 Produced By: Thyroid gland
• 📊 Measures: Active, unbound T4 in blood
• 📈 High: Hyperthyroidism, thyroid hormone resistance
• 📉 Low: Hypothyroidism, pituitary or hypothalamic disorders
• ✅ Normal Range: 0.8 – 1.8 ng/dL
• 🩺 Preferred marker for evaluating thyroid function, especially during pregnancy or when binding proteins are altered

3. Total T3

• ✅ Fasting: Yes
• 📍 Produced By: Thyroid and liver (T4 → T3 conversion)
• 📊 Measures: Both bound and free forms of T3
• 📈 High: Hyperthyroidism, T3 thyrotoxicosis
• 📉 Low: Hypothyroidism, severe illness
• ✅ Normal Range: 80 – 200 ng/dL
• 🩺 Useful when free T3 is not available or in borderline cases

4. Total T4

• ✅ Fasting: Yes
• 📍 Produced By: Thyroid
• 📊 Measures: Total amount of T4 (free + bound to proteins)
• 📈 High: Hyperthyroidism, increased binding proteins (pregnancy, estrogen use)
• 📉 Low: Hypothyroidism, low protein states
• ✅ Normal Range: 5 – 12 µg/dL
• 🩺 Influenced by binding proteins; best interpreted alongside Free T4 and TSH

5. TSH (Thyroid Stimulating Hormone)

• ✅ Fasting: Yes (especially in the morning)
• 📍 Produced By: Pituitary gland
• 📊 Measures: Pituitary’s signal to the thyroid to make hormones
• 📈 High: Hypothyroidism (thyroid not producing enough T3/T4)
• 📉 Low: Hyperthyroidism (too much hormone feedback)
• ✅ Normal Range: 0.4 – 4.0 mIU/L (may vary slightly by lab)
• 🩺 Primary screening test for thyroid disorders

🔁 How These Tests Work Together

🔬 When to Order All 5 Tests

• Persistent or ambiguous thyroid symptoms
• Nodules, goiter, or irregular thyroid medications
• Thyroid function in pregnancy or on estrogen therapy
• Suspected thyroid hormone resistance or binding protein abnormalities

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