Background: Hashimoto's thyroiditis (HT) has been found to coexist with differentiated thyroid cancer (DTC) in surgical specimens, but an association between the two conditions has been discounted by the medical literature. Therefore, we performed this study to determine any potential relationship between HT and the risk of developing DTC. Methods: We collected data for thyrotropin (TSH), thyroxine (T 4 ), thyroid peroxidase antibody (TPO-Ab) titers, surgical pathology, and weight-based levothyroxine (LT 4 ) replacement dose for patients who were referred for thyroid surgery. Patients with HT at final pathology were studied further. To estimate thyroid function, patients with preoperative hypothyroid HT (Hypo-HT) were divided into three equal groups based on their LT 4 replacement: LT 4 -Low (<0.90 lg/kg), LT 4 -Mid (0.90-1.43 lg/kg), and LT 4 -High (>1.43 lg/kg). A group of preoperatively euthyroid (Euth-HT) patients but with HT by pathology was also studied. All subjects were also grouped based on their TPO-Ab titer in TPO-high (titer >1:1000) or TPO-low/negative (titer <1:1000 or undetectable) groups. The relationship of HT and DTC was studied extensively. Results: Of 2811 subjects, 582 had HT on surgical pathology, 365 of whom were Euth-HT preoperatively. DTC was present in 47.9% of the Euth-HT, in 59.7% of LT 4 -Low, 29.8% of LT 4 -Mid, and 27.9% of LT 4 -High groups. The relative risk (RR) for DTC was significantly elevated for the Euth-HT and LT 4 -Low groups ( p < 0.001), but not for the LT 4 -Mid or LT 4 -High replacement dose groups. TPO-low/negative status conferred an increased RR in the Euth-HT and LT 4 -Low replacement dose groups ( p < 0.001 both), while TPO-high status decreased it in Euth-HT group ( p < 0.05) and made it nonsignificant in the LT 4 -Low group. Conclusions: HT pathology increases the risk for DTC only in euthyroid subjects and those with partially functional thyroid glands (LT 4 -Low) but not in fully hypothyroid HT (LT 4 -Mid and LT 4 -High). High TPO-Ab titers appear to protect against DTC in patients with HT.

BackgroundOur Thyroid-Multidisciplinary Clinic is a large referral site for thyroid diseases. Thyroid biopsies are mainly performed for thyroid cancer screening. Yet, Hashimoto thyroiditis (HT) is being too frequently diagnosed. The prevalence of HT is reported as 0.3-1.2% or twice the prevalence of type 1 diabetes. However, the prevalence of HT confirmed by cytology is still uncertain. To evaluate different aspects of thyroid physiopathology including prevalence of Hashimoto's, a database of clinical features, ultrasound images and cytology results of patients referred for FNA of thyroid nodules was prospectively developed.MethodsWe retrospectively studied 811 consecutive patients for whom ultrasound guided thyroid FNA biopsies were performed at our clinic over 2.5 year period (Mar/2006-Sep/2008).ResultsThe analysis of our database revealed that from 761 patients, 102 (13.4%) had HT, from whom 56 (7.4%) were euthyroid or had sub-clinical (non-hypothyroid) disease, and 46 (6%) were clinically hypothyroid.ConclusionsThis is the first study to show such a high prevalence of HT diagnosed by ultrasound-guided FNA. More strikingly, the prevalence of euthyroid HT, appears to be >5% similar to that of type 2 diabetes. Based on our results, there might be a need to follow up on cytological Hashimoto's to monitor for thyroid failure, especially in high risk states, like pregnancy. The potential risk for thyroid cancer in patients with biopsy-proven inflammation of thyroid epithelium remains to be established prospectively. However, it may explain the increased risk for thyroid cancer observed in patients with elevated but within normal TSH.