Triiodothyronine (T3) Test

The Free Triiodothyronine (FT3) test measures the level of unbound, biologically active T3 hormone in your blood. FT3 is one of the two main thyroid hormones (along with T4) and plays a key role in controlling metabolism, energy production, heart rate, and growth.

Why the FT3 Test Is Ordered

1. Evaluate thyroid function

• Suspected hyperthyroidism (overactive thyroid)
• Helps clarify thyroid status when FT4 and TSH are borderline

2. Monitor thyroid disease treatment

• Patients on thyroid hormone replacement (e.g., levothyroxine)
• Patients on anti-thyroid medications

3. Investigate abnormal TSH or FT4 levels

• FT3 is particularly useful when TSH is low but FT4 is normal (“T3-toxicosis”)

4. Assess symptoms of thyroid imbalance

• Hyperthyroidism symptoms: weight loss, rapid heartbeat, tremors, anxiety, heat intolerance
• Hypothyroidism symptoms: fatigue, weight gain, constipation, dry skin (FT3 less commonly low than FT4 in hypothyroidism)

Why FT3 Is Measured

• T3 is more active than T4 in tissues
• FT3 helps detect early or subtle thyroid overactivity

Clinical Use

• Diagnosis of hyperthyroidism
• Monitoring response to thyroid medication
• Assessing patients with thyroid disease symptoms but normal FT4
• Rarely used alone; usually interpreted with TSH and FT4

The FT3 test measures the active thyroid hormone and is most useful for detecting hyperthyroidism, evaluating complex thyroid cases, and monitoring thyroid therapy.