Serum thyroglobulin (Tg) levels are highly sensitive and specific tumor markers in patients with thyroid cancer who have undergone total thyroidectomy and radioiodine (RAI) therapy. Recently, we reported a case wherein serum Tg levels fluctuated according to hemoglobin A1c (HbA1c) levels, demonstrating a strong correlation between serum Tg and HbA1c levels. However, whether this association exists broadly in other patients with thyroid cancer remains unclear. Therefore, we retrospectively investigated this association in six patients with thyroid cancer and diabetes who underwent total thyroidectomy and RAI therapy at our institution. Two patients exhibited a significant correlation between serum Tg and HbA1c levels (r = 0.53, p <
0.01 and r = 0.66, p = 0.01). In these patients, a gradual decrease in serum Tg levels was observed, along with improved glycemic control. Two other patients showed a non-significant correlation between serum Tg and HbA1c levels (r = 0.72, p = 0.11 and r = 0.54, p = 0.17). In these patients, a rapid increase in serum Tg levels was observed, with HbA1c levels showing only small fluctuations. In the remaining two patients, no correlation was found
in these patients, the fluctuations in serum Tg levels were only small, despite fluctuations in HbA1c levels. In conclusion, serum Tg levels may be associated with HbA1c levels in some patients with thyroid cancer and diabetes. The correlation between serum Tg and HbA1c levels was more evident in patients with gradual fluctuations in Tg levels. Future studies with larger cohorts are necessary to clarify the underlying mechanism by which glycemic control influences Tg levels and establish appropriate monitoring methods.