A Case of Hypothyroidism with the Imapired Conversion of T4 to T3

• Published in: Japanese Journal of National Medical Services Vol. 49; no. 3; pp. 250 - 252
• Main Authors: MORITA Shigeki, MATSUDA Midori, ARIMA Tetsuhiko, ONO Keiji, MIZOKAMI Akinari, YAMAKAWA Kenichi
• Format: Journal Article
• Language: Japanese
• Published: Japanese Society of National Medical Services 1995; 一般社団法人 国立医療学会
• Subjects: hypothyroidism; low T3 syndrome; low T3症候群; MDS; T4 conversion; サイロキシン変換; 甲状腺機能低下症
• ISSN: 0021-1699; 1884-8729
• DOI: 10.11261/iryo1946.49.250

We report a case of hypothyroidism with unexpectedly low levels of serum T3 and free T3 probably due to impaired thyroxine conversion.The patient was 76 year-old blind male. His serum levels of T4, T3 and TSH were 0.5μg/dl, less than 0.5ng/ml and 44.9μIU/ml, respectively, and his illness was diagnosed as primary hypothyroidism in August 1991. With the increasing doses of thyroxine replacement (25 to 125μg/day), serum level of TSH decreased to 8.3μIU/ml and those of T4 and free T4 increased to normal ranges. However, serum level of T3 was still less than 0.5ng/ml and that of free T3 was also low (0.8pg/ml). He had not received iodide contrast media or taken excess iodide. Also he had no hypo-albuminemia, drugs inducing impaired thyroxine-metabolism, anti-T3 antibody, TBG deficiency or low T3 syndrome. The etiology of low level of serum T3 of this patient may be due to the impaired conversion (deiodination) of T4 to T3. 今回我々は, 血中T3・遊離T3が異常低値であった甲状腺機能低下症の1例を経験し, その成因として, サイロキシン変換障害の可能性が考えられたため, 報告する.症例は76歳男性. 盲目. 平成3年8月, 血中T4=0.5μg/dl, T3=0.5ng/ml以下, TSH=44.9μIU/mlであったため, 原発性甲状腺機能低下症と診断した. サイロキシンの漸増(25から125μg/日)に伴い, TSHは8.3μIU/mlまで減少し, 血中T4・遊離T4は正常範囲まで増加したが, 血中T3は0.5ng/ml以下のままであり, 血中遊離T3も0.8pg/mlと低値であった. 造影剤の使用, ヨードの過剰摂取, 低アルブミン血症, サイロキシンの代謝障害をきたす薬剤の使用などはなく, 血中抗T3抗体は陰性であった. TBG減少症やlow T3症候群によるものは考えにくく, 本例の血中T3低値の原因として, T4からT3への変換(脱ヨード)障害の可能性が考えられた.