Background: Although thyroid dysfunction is associated with anemia, data on the association between thyroid function and anemia in the euthyroid range are scarce. We aimed to evaluate the association between anemia and serum free T4 (fT4) and thyrotropin (TSH) levels in euthyroid adults.
Methods: Data of participants aged ≥19 years were obtained from the Korea National Health and Nutrition Examination Survey VI (2013–2015), which is a nationwide, population-based study. Anemia was defined as hemoglobin (Hb) <13 and <12 g/dL for men and women (<11 g/dL for pregnant women), respectively.
Results: Overall, 5,368 patients were included, of whom 6.2% had anemia, and more women (10.0%) had anemia than men (3.2%; P < 0.001). In the multivariate analysis, serum fT4 levels, but not TSH levels, had an independently positive association with serum Hb levels in both men (P < 0.001) and women (P < 0.001). When we categorized participants into fT4 quartiles, those in the lowest quartile had 4% and 3% lower serum Hb levels than those in the highest quartile among men and women, respectively. In both men and women, serum Hb levels linearly reduced across decreasing serum fT4 quartile groups (P < 0.001 and P < 0.001, respectively). Participants with low-normal fT4 had 5.6 (P < 0.001) and 15.5 times (P < 0.001) higher risks for developing anemia than those with high-normal fT4 among men and women, respectively.
Conclusions: A low-normal serum fT4 level was associated with a lower serum Hb level and a higher risk of anemia in euthyroid adults. These results suggest that the reference range of fT4 is not optimal with respect to anemia and that avoiding low-normal serum fT4 levels may help avoid anemia in both men and women.