Adrenal insufficiency influences the pattern of thyrotoxicosis toward triiodothyronine predominance.

Serum-free thyroxine (FT4) levels are typically higher than free triiodothyronine (FT3) levels at the onset of thyrotoxicosis in patients with painless thyroiditis, reflecting the high ratio of stored thyroid hormone in the thyroid gland. We report a case of a 58-year-old man with painless thyroiditis and adrenal insufficiency caused by isolated adrenocorticotropin (ACTH) deficiency (IAD). Upon admission, the FT3 level was high, while the FT4 level was within the normal range. Likewise, the FT3/FT4 ratio was high. Therefore, the diagnosis of triiodothyronine (T3) thyrotoxicosis was made. He was diagnosed with painless thyroiditis because he did not have neck pain, and 99mTechnetium-thyroid scintigraphy revealed low thyroid uptake. The FT3/FT4 ratio fell after the increased serum cortisol level via daily administration of tetracosactide acetate. The elevated FT3/FT4 ratio in this patient could be explained by increased T4 to T3 conversion in a low-cortisol state. It has been suggested that destructive thyroiditis, accompanied by a high FT3/FT4 ratio, is a valuable indicator of the accompaniment of adrenal deficiency.