A 25-year-old man was hospitalized for weight loss, abdominal pain and palpitations. Diagnosis of hyperthyroidism caused by Graves’ disease was made and methimazol was started. During biological monitoring, despite decreasing methimazol dosage and the addition of L-thyroxine, the patient kept an atypical hormonal profile with very low T4, and normal or slightly elevated T3 and TSH [T4 0.19ng/dL (N: 0.8–1.7) T3 3.8pg/mL (1.8–4.6), TSH 9.5mIU/L (N: 0.4–4)]. Given this profile, persistent fatigue and abdominal pain, pituitary failure was suspected but a basal morning cortisol of 260ng/mL and a normal TRH stimulation test excluded pituitary insufficiency. Finally, we kept the diagnosis of T3-predominant Graves’ disease. Since thyroid function was difficult to normalize, our patient underwent total thyroidectomy 9 months after the diagnosis.

T3-predominant Graves’ diseases are characterized by the rising of T3/T4 ratio emerging only after the onset of antithyroid drugs and are associated with particularly elevated TSI levels. High T3/T4 ratio seems to be linked to an increase in the activity and/or the expression of type 1 and type 2 iodothyronine deiodinases. Usual T3/T4 ratios (pg/mL/ng/dL) described in the literature in T3-predominant Graves’ diseases fluctuate between 4 and 10 (N: 1.8–3.3). Here, we observed much higher T3/T4 ratios (up to 20) associated with particularly low T4 levels. Rising of the T3/T4 ratio at the beginning of antithyroid drugs and normalization after thyroidectomy on L-Thyroxine therapy suggests a role of antithyroid drugs in the activation of iodothyronine deiodinases.