The serum thyrotropin (TSH) level is the most sensitive index of thyroid function in apparently healthy subjects. The ongoing debate over subclinical hypothyroidism, its association with adverse health outcomes, recommendations for screening, and the use of levothyroxine for treatment has maintained the focus on defining the upper limit of the normal range of serum TSH levels.Recent data from the United States have shown a progressive increase in TSH levels with age [1][2][3]. On the other hand, data from Germany, where the iodine supply is deficient, yielded lower TSH reference levels that declined with age [4][5][6].In Japan, iodine intake is more than sufficient because of a diet that is rich in seaweed. Iodine intake may have an impact on the reference serum TSH concentraReference limits for serum thyrotropin in a Japanese population Ai Yoshihara, Jaeduk Yoshimura Noh, Hidemi Ohye, Shiori Sato, Kenichi Sekiya, Yuka Kosuga, Miho Suzuki, Masako Matsumoto, Yo Kunii, Natsuko Watanabe, Koji Mukasa, Kunihiko Ito and Koichi ItoIto Hospital, Tokyo, Japan Abstract. The aim of the present study was to establish new reference intervals for serum thyrotropin (TSH) levels in Japanese subjects without antithyroid antibodies. We reviewed the serum TSH level of all patients 20 years of age and over who attended the outpatient clinic of our hospital between January 1, 2003, and September 20, 2010. The thyroid gland of every patient was examined by ultrasonography, and subjects found to have a normal thyroid were chosen. The following subjects were excluded: subjects with past history of thyroid diseases; subjects whose serum was positive for antithyroid antibodies; pregnant women; patients taking medication that might affect their free thyroxine (fT 4 ) level or TSH levels. Ultimately, 1388 subjects were included in the reference population. The serum TSH levels shifted to higher ranges as the age of the groups increased. The calculated reference range was 0.39-4.29 mIU/L in the 20-29-year-old group, 0.34-3.90 mIU/L in the 30-39-year-old group, 0.56-5.02 mIU/L in the 40-49-year-old group, 0.51-5.30 mIU/L in the 50-59-year-old group, 0.60-4.85 mIU/L in the 60-69-year-old group, 0.62-6.15 mIU/L in the over 70-year-old group. The results of this study showed that the upper limit of the normal range of serum TSH levels increased with age in a Japanese population. Since the number of elderly reference subjects was relatively small, further study is needed. Setting the age-and racespecific reference limits for serum TSH levels is important in order to prevent significant misclassifications of patients with abnormal TSH levels.Key words: Thyrotropin, Reference limits, Antithyroid antibodies, Age, Ultrasonography tions of the population of a country. We reviewed the serum TSH levels of patients aged 20 years and over who had attended our outpatient clinic. All patients underwent thyroid ultrasonography and blood tests, including measurement of their serum thyroid hormone levels and testing for the presence of antithyroid antibodies....