Tri-iodothyronine (T3) is the main active hormone, and 20% of this is derived from the thyroid gland and 80% is from the peripheral tissue according to 5'-monodeiodination of thyroxine (T4). In the previous studies, normal T3 levels were achieved with traditional levothyroxine (LT4) therapy alone in athyreotic patients, but there has been no data about the factors influencing peripheral conversion of LT4. The aim of this study was to determine the factor(s) influencing peripheral conversion of

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... T4 to T3 in athyreotic patients during LT4 replacement. Methods: The patients who underwent total-thyroidectomy for any cause, and mostly for thyroid cancers, at Asan Medical Center between 2000 and 2008 were enrolled. The free T4, T3 and thyroid stimulating hormone (TSH) levels and age, gender, weight, height, body mass index (BMI) and the T4 dose were measured. Only patients with normal ranges of free T4 and TSH were included in the analysis. Results: A total of 143 patients were enrolled. The mean T3, free T4 and TSH levels were 143.7 ng/dL, 1.4 ng/dL and 1.6 µU/mL, respectively. The mean weight and BMI were 62.9 kg and 24.6 kg/m 2 , respectively. We divided them into two groups according to the serum T3 level and we compared the characteristics of the groups. There were no differences in age, the gender distribution, the T4 dose/weight and the BMI between the low T3 group (T3 ≤ 122 ng/dL, n = 14) and the normal T3 group (T3 > 122 ng/dL, n = 129). In the low T3 group, the mean body weight was significantly lower than that of the normal T3 group (59.0 ± 6.0 vs. 63.4 ± 9.9, respectively, P = 0.025). Conclusion: Lean body mass seems to be an important factor for determining the peripheral conversion of T4 to T3 in human. This suggest that a combination of T3/T4 is better than T4 only when we treat the patients with hypothyroidism and who have a negligible amount of functioning thyroid tissue, if they have a low lean body mass. (Endocrinol Metab 25:119-124, 2010) ORIGINAL ARTICLE 서 론 말초조직에서 갑상선호르몬의 생물학적 활성도는 삼요오드티로 닌(Tri-iodothyronine, T3)이 티록신(Thyroxine, T4)보다 3-4배 정도 강하며 주로 T3가 조직에 존재하는 핵 내 갑상선호르몬 수용체와 결합하여 갑상선호르몬의 생물학적 작용을 나타낸다. 혈청 T3의 20%는 갑상선에서 생성된 것이며 80%는 말초조직에서 T4가 탈요 오드화에 의해 T3로 전환된 것이다. 갑상선기능저하증 환자 치료 시 T4만 투여하여도 말초조직에서 T4가 T3로 전환되므로 일반적으로 T4만 투여하여도 무방한 것으로 알려져 있다. 따라서 현재 갑상선기 능저하증 환자에서 갑상선호르몬 보충요법으로 레보티록신(Levothyroxine, LT4)을 표준 치료제로 사용하고 있으며 생체 내에서 LT4 에서 T3로의 말초변환 및 이의 조절은 이전 연구들에서 증명되었다 [1-3]. 그러나, 이론적으로 갑상선 조직이 수술이나 질환 등으로 감소