Searching for the most effective thyrotropin (TSH) threshold to rule-out autonomously functioning thyroid nodules in iodine deficient regions

The purpose of this study is to evaluate the distribution of thyrotropin (TSH) values in patients with autonomously functioning thyroid nodules and to set a TSH threshold above which thyroid scintigraphy would be obviated. Four hundred fifty one patients were included in the present study. Inclusion criteria were age > 18 years, TSH levels between 0.40 and 4.0 mIU/L, detection of a single solid or predominantly solid thyroid nodule >10 mm in the longest diameter. Thyroid ultrasound and thyroid scintigraphy with ^99mTc-pertechnetate were performed concurrently in all patients. Among 451 enrolled patients, 173 (38 %) had an autonomously functioning thyroid nodules, of which 137 (79 %) with a normal TSH level. Demographic data and nodules’ volume were not significantly different in patients with autonomously functioning thyroid nodules and non-functioning nodules, respectively. However, TSH levels were nonetheless significantly lower in patients with autonomously functioning thyroid nodules compared to those with non-functioning nodules (p < 0.001). Adopting a TSH cutoff level at 2.38 mUI/L, all autonomously functioning thyroid nodules were correctly identified (i.e., 100 % sensitivity) with a 100 % negative predictive value. Our study showed a very high prevalence of autonomously functioning thyroid nodules in mildly iodine-deficient regions and confirmed that serum TSH is not an effective screening test to diagnose an autonomously functioning thyroid nodules. Our data add arguments in favor of the first-line use of thyroid scintigraphy to assess thyroid nodules, at least in iodine deficient areas. As all scintigraphically detected autonomously functioning thyroid nodules had a TSH level below 2.38 mUI/L, a thyroid scintigraphy should be omitted when higher TSH values are found in patients carrying a thyroid nodule.