Introduction: Non-diagnostic rates for thyroid fine needle aspiration vary considerably, but reasons for this variation are poorly understood. Methods: Our group reviewed the results of all thyroid aspirations with surgical resection for the last 13 years at our institutions, combined these with those in the literature, and correlated non-diagnostic rates with positivity rates. Results: Non-diagnostic rates were negatively correlated with positivity rates (R2 = 0.55). Studies that had significantly lower non-diagnostic rates than the rest of the literature (2 vs. 15%, p = 0.002) had different or undefined adequacy criteria or used core biopsies in addition to fine needle aspiration. Conclusions: Non-diagnostic rates for thyroid fine needle aspirations are negatively correlated with positivity rates. These data provide more realistic estimates of achievable adequacy rates.