Differential Diagnosis of Decreased Free T4 with Increased Free T3

Primary Diagnostic Considerations

In Graves' disease, enhanced peripheral conversion of T4 to T3 can create a pattern where T3 rises disproportionately, potentially depleting T4 stores, according to the American College of Endocrinology 1
Confirming Graves' disease involves checking thyroid receptor antibodies (TRAb) or thyroid stimulating immunoglobulin (TSI) testing if clinical features suggest the disease, as recommended by the American Thyroid Association 1, 2
Clinical symptoms to assess in suspected Graves' disease include weight loss, palpitations, heat intolerance, hyperactivity, and tremor, as outlined by the American College of Endocrinology 2

Certain physiologic or pathologic states can upregulate type 1 deiodinase (DIO1), increasing conversion of T4 to T3, as noted in Clinical Gastroenterology and Hepatology 3
Iron deficiency can impair T4 synthesis and hepatic conversion of T4 to T3, but typically results in low T3, not high T3, according to Sports Medicine 4

If TSH is suppressed (<0.1 mIU/L), proceed with hyperthyroidism workup, including checking thyroid receptor antibodies (TRAb/TSI) and evaluating for clinical hyperthyroid symptoms, as recommended by the American College of Endocrinology 1, 2
Unusual presentations with elevated T3 and normal or low TSH require endocrinology referral, according to the American College of Endocrinology 2