Unit Converter
Triiodothyronine free (FT3)
(Biologically Active Unbound T3 - Sensitive Indicator of Thyroid Hormone Excess & Peripheral Conversion)
Synonyms
- Free T3
- FT3
- Unbound T3
- Dialyzable T3 (reference method)
- Non-protein-bound T3
Units of Measurement
- pmol/L
- pg/mL
- pg/dL
- pg/100 mL
- pg%
- ng/dL
- ng/L
Unit Conversions
Molecular Weight of T3 = 650.97 g/mol
pmol/L ↔ pg/mL
1 pmol/L=0.651 pg/mL1\ \text{pmol/L} = 0.651\ \text{pg/mL}1 pmol/L=0.651 pg/mL 1 pg/mL=1.54 pmol/L1\ \text{pg/mL} = 1.54\ \text{pmol/L}1 pg/mL=1.54 pmol/L
pmol/L ↔ ng/dL
1 ng/dL=15.4 pmol/L1\ \text{ng/dL} = 15.4\ \text{pmol/L}1 ng/dL=15.4 pmol/L 1 pmol/L=0.065 ng/dL1\ \text{pmol/L} = 0.065\ \text{ng/dL}1 pmol/L=0.065 ng/dL
pg/mL ↔ ng/L
1 pg/mL=1 ng/L1\ \text{pg/mL} = 1\ \text{ng/L}1 pg/mL=1 ng/L
pg/dL = pg% = pg/100 mL
(Older volumetric units; equal to one another.)
Description
Free T3 represents the ~0.3% unbound fraction of circulating triiodothyronine that is:
- Not bound to TBG
- Not bound to albumin
- Not bound to prealbumin/transthyretin
Because FT3 reflects the active hormone available to tissues, it is a sensitive marker of:
- Hyperthyroidism
- Peripheral conversion of T4 → T3
- Thyroid hormone resistance
FT3 is less reliable for diagnosing hypothyroidism than FT4, but highly useful for detecting T3-toxicosis.
Physiological Role
FT3 controls:
- Basal metabolic rate
- Sympathetic drive
- Thermogenesis
- Cardiac output & heart rate
- Lipid and carbohydrate metabolism
- Brain and bone development
- GI motility
- Mood & cognition
FT3 is 3–4 times more potent than FT4.
Clinical Significance
ELEVATED FT3 (T3-Toxicosis / Hyperthyroidism)
Most common causes
1. Graves’ Disease
Often shows disproportionate rise in FT3 (T3-predominant thyrotoxicosis).
2. Toxic Multinodular Goiter
3. Toxic Adenoma
4. Thyroiditis
5. Excess T3 or Levothyroxine Intake
Intentional or accidental.
6. TSH-Secreting Pituitary Adenoma
High FT3 + high TSH.
7. Thyroid Hormone Resistance
High FT3 + normal/high TSH.
LOW FT3
1. Non-Thyroidal Illness Syndrome
Most common cause of low FT3 worldwide.
Seen in:
- Sepsis
- Trauma
- ICU/critical illness
- Liver failure
- Renal failure
- Malnutrition
2. Hypothyroidism
FT3 often preserved until late stages.
3. Certain Medications
- Amiodarone
- Propranolol
- Steroids
- Lithium
4. Chronic liver disease
Reduced conversion of T4 → T3.
Reference Intervals
(Tietz 8E + ATA/AACE + Mayo + ARUP)
Free T3
- 3.0 – 6.0 pmol/L
(= 2.0 – 4.0 pg/mL)
(= 0.20 – 0.39 ng/dL)
Clinical Interpretation
- FT3 > 6.0 pmol/L → hyperthyroidism likely
- FT3 > 10 pmol/L → marked thyrotoxicosis
- FT3 < 3.0 pmol/L → low T3 / NTIS / hypothyroidism
- FT3 < 2.0 pmol/L → severe illness or strong NTIS pattern
Diagnostic Uses
1. Evaluate Hyperthyroidism
Most sensitive marker of thyroid hormone excess.
2. Diagnosis of T3-Toxicosis
High FT3 with normal FT4.
3. NTIS (Non-Thyroidal Illness)
Earliest change → fall in FT3.
4. Monitor Therapy
Useful in patients on combination LT4+LT3 therapy.
5. Thyroid Hormone Resistance
High FT3 with normal or high TSH.
Analytical Notes
- Best measured by equilibrium dialysis FT3 (gold standard).
- Routine immunoassays for FT3 are variable and less reliable than FT4.
- Biotin supplements can cause false high FT3 in many assays.
- Heparin infusion may artifactually raise FT3 (lipase-driven FFA displacement).
- Assay interference is more common in FT3 than FT4.
Clinical Pearls
- In Graves’ disease, FT3 rises more than FT4.
- Elderly patients may have “normal FT3 hyperthyroidism” (apathetic hyperthyroidism).
- NTIS → low FT3, normal/low FT4, normal/low TSH - do not treat the thyroid.
- FT3 is useful when FT4 and TSH are borderline but symptoms suggest thyrotoxicosis.
- Total T3 sometimes correlates better than FT3 in mild hyperthyroidism due to assay issues.
Interesting Fact
Although FT3 is present at extremely low concentrations (~0.3% of total T3), it mediates nearly all of thyroid hormone metabolic effects, making it the “active currency” of thyroid physiology.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Thyroid Hormones
- ATA / AACE Thyroid Dysfunction Guidelines
- European Thyroid Association (ETA)
- Mayo Clinic Laboratories - FT3
- ARUP Consult - Thyroid Function
- NIH / MedlinePlus - Triiodothyronine