Triiodothyronine (T3) is one of the two main thyroid hormones, the other being thyroxine (T4).

The free fraction (FT3) represents the biologically active hormone not bound to proteins.

FT3 plays a critical role in metabolism, growth, and energy regulation.

Clinical uses include:

Diagnosing hyperthyroidism (overactive thyroid).

Assessing hypothyroidism (underactive thyroid).

Monitoring patients on thyroid hormone replacement therapy.

Evaluating pituitary or hypothalamic disorders affecting thyroid function.

Often measured alongside FT4 and TSH for a complete thyroid profile.

Analyte: Serum free triiodothyronine (FT3) concentration.

Normal Range (varies by lab): ~2.0–4.4 pg/mL.

Abnormal Findings:

High FT3: Suggests hyperthyroidism, thyroid hormone overmedication, or thyroid nodules.

Low FT3: Indicates hypothyroidism, pituitary/hypothalamic dysfunction, or severe illness (low T3 syndrome).

Clinical Use:

Provides a direct measure of active thyroid hormone.

Helps differentiate causes of thyroid dysfunction.

Supports monitoring of thyroid therapy and metabolic status.