Objective: The thyroid gland has two different endocrine cell populations, namely, follicular cells and parafollicular cells. Follicular cells responsible for secreting triiodothyronine (T₃) and thyroxine (T₄), hormones that control the metabolism. Parafollicular cells produce calcitonin, a hypocalcemic and hypophosphatemic horm one. The aim of our study is to examine the relationship between the follicular and parafollicular cells.

Materials and Methods: In our study, between January 2009-January 2010, patients who are diagnosed with thyroid cancer were evaluated retrospectively. Patients with thyroid cancer were divided into two study groups, according to the calcitonin level. 30 patients with normal calcitonin levels and 30 patients with elevated calcitonin levels were enrolled in the study. Calcitonin, T₃, T ₄, free T₃ (FT₃), free T₄ (FT₄), thyroid-stimulating hormone (TSH) levels were measured in whole blood by chemiluminescence method.

Results: In the group with normal calcitonin levels and in the group elevated calcitonin levels, calcitonin levels, respectively, 2.57±1.53 pg / mL and 265.56±122.49 pg / mL. FT₃, FT₄, T₃ and T ₄ levels in the group with elevated calcitonin levels were found to be significantly higher compared with the the group with normal calcitonin levels (respectively, p=0.025, p=0.007, p=0.026, p=0.008). In the group with elevated calcitonin levels, TSH levels was significantly lower than the group with normal calcitonin levels (p<0.0001). Also, a significant negative correlation was found to be existing between the calcitonin levels and TSH levels in group with elevated calcitonin levels (rs = -0.51, p<0.0001).

Conclusion: The existence of high levels of thyroid hormones in patients with high serum calcitonin levels suggests a possible functional interaction between follicular and parafollicular cells and these two cell groups may not be completely independent from each other.