Background: The introduction of highly sensitive imaging techniques has made it possible to detect many non-palpable thyroid nodules with size less than 1 centimetre. The prevalence of these lesions appears to be high in the general population and increases in iodine deficiency areas. Fine needle aspiration biopsy (FNAB) is considered the most reliable test for the diagnosis of thyroid nodules.
Methods: From January 1991 to December 2000, 6627 nodules were referred for FNAB to our department. In 4871 (73.5%) euthyroid patients the mean daily urinary excretion of iodine was 73+/-18 micromg/day.
Results: In 2401 nodules (36.2%) the diameter was less than one centimetre. Histologic confirmation of malignancy was available in 187 (89.9%) of 208 operated nodules with diameter more than 1 cm and in 64 (78%) of 82 with diameter less than 1 cm. Logistic regression analysis indicated that a solid hypoechoic feature was a useful criterion to predict malignancy (p<0.001), as well as the presence of calcification (p<0.5) and blurred margins (p<0.5). Lymph node metastases were present in 24.3% of carcinomas with diameter more than 1 cm but, surprisingly, also in 20.3% of those with diameter less than 1 cm. No correlation was seen between diameter of carcinoma nodules and presence of lymph node metastasis.
Conclusions: These data suggest that the potential malignancy of thyroid nodules is not only correlated to the diameter but to other parameters such as the biological difference of thyroid cancer that could be hypothesized in an iodine deficiency area. In conclusion, FNAB is recommended for nodules with diameter less than 1 centimetre when sonographic findings suggest malignancy such as a hypoechoic pattern, an irregular margin or internal microcalcification.