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Original Article Factors Influencing Peripheral Conversion of Thyroxine to Tri-Iodothyronine in Athyreotic Individuals during Levothyroxine Replacement.
Eui Young Kim, Won Gu Kim, Tae Yong Kim, Jong Ho Yoon, Suck Joon Hong, Young Kee Shong, Won Bae Kim
Endocrinology and Metabolism 2010;25(2):119-124
DOI: https://doi.org/10.3803/EnM.2010.25.2.119
Published online: June 1, 2010
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1Department of Endocrinology & Metabolism, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea. kimwb@amc.seoul.kr
2Department of Surgery, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea.

BACKGROUND
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 LT4 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 microU/mL, respectively. The mean weight and BMI were 62.9 kg and 24.6 kg/m2, 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 < or = 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.

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