1. Introduction to Free Triiodothyronine (FT3)

What is Free Triiodothyronine? Free Triiodothyronine (FT3) is the unbound, biologically active fraction of triiodothyronine (T3), a thyroid hormone produced by the thyroid gland and converted from thyroxine (T4) in peripheral tissues. FT3 is more potent than T4 and regulates metabolism, heart rate, and body temperature. Serum FT3 levels are measured to diagnose thyroid disorders, particularly hyperthyroidism, assess thyroid function in complex cases, or monitor treatment responses, such as in thyroid hormone replacement or antithyroid therapy.

2. What is a Free Triiodothyronine Unit Converter?

Definition: The Free Triiodothyronine Unit Converter transforms serum FT3 concentrations between units like pmol/L, pg/mL, pg/dL, pg/100mL, pg%, ng/dL, and ng/L, standardizing results for clinical use.

Purpose: It enables clinicians and researchers to interpret FT3 levels consistently, aiding in the diagnosis and management of thyroid-related conditions.

3. Importance of Free Triiodothyronine Unit Conversions

Unit conversions for FT3 are essential for:

• Standardization: Labs use pmol/L as the SI unit, but pg/mL is common in some regions; conversions ensure uniform interpretation across protocols.
• Diagnosis and Monitoring: Accurate conversions help identify thyroid dysfunction, particularly in hyperthyroidism, and monitor treatment efficacy, such as antithyroid drugs or T3 therapy.
• Research: Enables consistent comparison of FT3 data across studies, improving reliability in endocrinology research.

4. Clinical Significance

Elevated Levels: High FT3 levels (>6.8 pmol/L or >4.4 pg/mL) may indicate:

• Hyperthyroidism: Excessive T3 production (e.g., Graves’ disease, toxic multinodular goiter) causes symptoms like palpitations, weight loss, and anxiety.
• T3 Toxicosis: Isolated FT3 elevation with normal FT4, often seen in early hyperthyroidism or toxic adenoma.
• Thyroiditis: Inflammation (e.g., subacute or silent thyroiditis) can release stored T3, temporarily elevating FT3.
• Exogenous T3: Overdose of T3-containing medications (e.g., liothyronine) can increase FT3 levels.

Elevated FT3 requires correlation with TSH, FT4, and clinical symptoms to confirm hyperthyroidism or other causes.

Low Levels: Low FT3 levels (<3.1 pmol/L or <2.0 pg/mL) may indicate:

• Hypothyroidism: Reduced thyroid hormone production (e.g., Hashimoto’s thyroiditis) causes fatigue, weight gain, and depression, though FT3 is less sensitive than FT4 for diagnosis.
• Non-Thyroidal Illness Syndrome: Severe illness (e.g., sepsis, critical care) suppresses T3 conversion, lowering FT3 without true thyroid dysfunction (euthyroid sick syndrome).
• Secondary Hypothyroidism: Pituitary or hypothalamic dysfunction reduces TSH, indirectly lowering FT3.
• Medications: Drugs like amiodarone, propylthiouracil, or high-dose glucocorticoids can decrease FT3 levels.

Low FT3 requires correlation with TSH, FT4, and clinical history to distinguish thyroidal, non-thyroidal, or medication-related causes.

Normal Ranges and Conversions:

• Adults (serum FT3): 3.1–6.8 pmol/L (2.0–4.4 pg/mL).
• Children: 3.8–8.0 pmol/L (2.5–5.2 pg/mL), varying by age.
• Pregnancy: 2.8–5.5 pmol/L (1.8–3.6 pg/mL), slightly lower due to binding protein changes.
• Conversions: 1 pmol/L = 0.0651 pg/mL (based on triiodothyronine’s molecular weight of ~650.97 g/mol); 1 pg/mL = 15.36 pmol/L; 1 pg/dL = 0.1536 pmol/L; 1 pg/100mL = 1 pg% = 1.536 pmol/L; 1 ng/dL = 1.536 pmol/L; 1 ng/L = 0.001536 pmol/L.
• Interpretation: Low: <3.1 pmol/L (<2.0 pg/mL); Normal: 3.1–6.8 pmol/L (2.0–4.4 pg/mL); High: >6.8 pmol/L (>4.4 pg/mL). Ranges vary by lab, assay, and population (e.g., pregnancy)—consult a healthcare provider.

5. Precautions

Pre-Test Precautions:

• No fasting is typically required, but collect samples in the morning for consistency, as FT3 may show minor diurnal variation.
• Disclose medications (e.g., liothyronine, amiodarone, biotin) or conditions (e.g., pregnancy, critical illness) that may affect FT3 levels or assay accuracy.
• Use serum samples; process promptly to ensure stability, as FT3 is relatively stable but can be affected by improper storage.
• Avoid biotin supplements for 24–48 hours before testing, as high doses can interfere with thyroid hormone assays.

Post-Test Precautions:

• Apply pressure to the venipuncture site for 3–5 minutes to ensure hemostasis; avoid rubbing to prevent bruising.
• Extend pressure for patients with bleeding tendencies.
• If dizziness occurs post-draw, lie down, hydrate, and rest until stable.
• For bruising, apply a warm towel after 24 hours to aid absorption.

Analytical Interferences:

• Biotin, heterophile antibodies, or binding protein abnormalities (e.g., in pregnancy or liver disease) can interfere with FT3 assays; use specific methods (e.g., equilibrium dialysis) for accuracy.
• Correlate FT3 with TSH, FT4, and clinical history to confirm thyroid dysfunction—consult a healthcare provider for accuracy.

6. Frequently Asked Questions (FAQ)

Q: Why are there different units for FT3?
A: Units like pmol/L are the SI standard, while pg/mL is common in some regions; conversions ensure consistency across labs and protocols.

Q: What is a normal FT3 level?
A: Typically 3.1–6.8 pmol/L (2.0–4.4 pg/mL) for adults. Consult a healthcare provider for personalized interpretation.

Q: How does elevated FT3 impact health?
A: High FT3 may indicate hyperthyroidism or T3 toxicosis, causing symptoms like palpitations, weight loss, or tremor, often linked to Graves’ disease or thyroiditis.

Q: Can this converter be used for other thyroid hormones?
A: No, it’s specific to FT3; other hormones (e.g., FT4, TSH) have different molecular weights or measurement units—use appropriate converters.