From the Guidelines
Thyroid function tests, including Free T3, Total T3, Reverse T3, and T3 uptake, are essential for assessing thyroid gland activity and diagnosing conditions like hypothyroidism and hyperthyroidism. These tests measure various forms of thyroid hormones in the blood, with Free T3 being the unbound, biologically active form of triiodothyronine that directly affects metabolism in tissues 1. Total T3 measures both bound and unbound T3, providing an overall picture of T3 levels. Reverse T3 is an inactive form of T3 that increases during illness or stress, indicating problems with T3 utilization. T3 uptake is an indirect measurement that helps evaluate thyroid hormone binding capacity in the blood, not actual T3 levels.
Key Points to Consider
- Thyroid function tests are typically ordered together with TSH (thyroid stimulating hormone) and T4 tests for comprehensive thyroid evaluation 1.
- Interpretation requires considering all values together, as individual results can be misleading.
- Normal ranges vary by laboratory, age, and gender.
- These tests help diagnose conditions like hypothyroidism, hyperthyroidism, and monitor treatment effectiveness.
- Certain medications, supplements, and medical conditions can affect results, so fasting may be required before testing, and patients should inform their doctor about all medications they're taking to ensure accurate interpretation.
Clinical Recommendations
- Consider testing individuals with type 1 diabetes for antithyroid peroxidase and antithyroglobulin antibodies soon after the diagnosis 1.
- Measure thyroid-stimulating hormone concentrations at diagnosis when clinically stable or soon after glycemic control has been established.
- If normal, consider rechecking every 1–2 years or sooner if the patient develops symptoms suggestive of thyroid dysfunction, thyromegaly, an abnormal growth rate, or an unexplained glycemic variation.