We report a case of type 2 diabetes mellitus presenting hypothyroidism due to overeating of seaweed that was noticed as a result of a discrepancy between glycated albumin (GA) and glycated hemoglobin (GHb). A 71-year-old woman was undergoing managed treatment with oral medicines and insulin for diabetes mellitus with no sign of thyroid disease. Her thyroid function was euthyroid without aid of thyroid hormone. All of the patient's thyroid autoantibodies were negative. Fifteen weeks prior to indications of hypothyroidism, she had started to consume large amounts (100-200 g dry weight equivalent) of cooked "wakame" seaweed every morning. Just before admission to our hospital, her GA was 26.9%, while GHb and fasting plasma glucose remained within normal ranges (less than 5.6%, and 106 mg, respectively). This discrepancy between GA and GHb drew our attention to the development of complications. Naive interview of the patient led us to believe a thyroid hormone deficiency existed, though without any related complaints or findings, such as non-pitting edema, cold intolerance, or easy fatiguing. Seaweed consumption was stopped and periodic observation of thyroid function started. As thyroid hormone levels moved into normal range, GA and GHb returned to their normal relative ratio after 3 months. Thus, measurement of the relative ratio of GA and GHb may be useful for glycemic monitoring, with the potential as a readily available glycemic control marker for patients with changeable complications.