Objective: Based on the assumption that normal TSH concentration rules out the presence of autonomous functioning thyroid nodules (AFTNs), clinical guidelines on the management of thyroid nodules only recommend a thyroid scan if TSH concentration is subnormal. However, the proportion of AFTN presenting with a normal TSH is unknown. Our objective is therefore to determine the proportion of AFTNs with a normal TSH level to ascertain whether a normal TSH really rules out an AFTN.
Design: Retrospective study on 368 patients with an AFTN.
Methods: Thyroid scans with a diagnosis of AFTN were reviewed retrospectively by one of us (R Moreno-Reyes), blinded to the clinical data. The diagnosis of solitary AFTN was confirmed in 368 patients. Among them, we selected 217 patients based on the absence of another thyroid nodule >10 mm, the absence of medical conditions able to interfere with thyroid function, and the completeness of the data.
Results: The proportion of AFTNs with normal TSH was 49%. This proportion increased to 71% in patients for whom thyroid scan was performed in the workup of a thyroid nodule.
Conclusions: Our data suggest that serum TSH is not an effective screening tool to diagnose AFTNs. Using 'TSH-only' screening, as recommended by the majority of guidelines, the diagnosis of AFTN would have been missed in 71% of our patients in the workup of a thyroid nodule. Thyroid scan remains the gold standard for detecting AFTN and should be considered before performing fine-needle aspiration cytology (FNAC), as the reliability of FNAC in an unsuspected AFTN remains unclear.