Free T3

CPT Code


Specimen Type


Container Type

Serum Separator Tube (SST)/Yellow


Room Temperature, < 8 hours; 2-8°C, < 48 hours; -20°C, > 48 hours

Handling Instructions

Allow to clot for 30 mins, centrifuge within 2 hours

Clinical Notes

T3 levels are particularly helpful in the evaluation of clinical hyperthyroidism when the TSH is low but total or free T4 levels are normal. In this situation, an elevated T3 level (free or total) supports a diagnosis of T3 thyrotoxicosis, which represents 5% of hyperthyroid cases. While total T3 is more commonly ordered due to better validation, free T3 is preferable when there is clinical concern that the quantity of carrier protein in the blood may be abnormal, which affects interpretation of total T3. Carrier protein (thyroxine binding globulin (TBG), prealbumin, and albumin) levels may be elevated in clinical conditions such as pregnancy, or low in conditions such as malnutrition.