Livingston M, Birch K, Guy M, Kane J, Heald A H
Br J Biomed Sci. 2015;72(4):160-3. doi: 10.1080/09674845.2015.11665746.
Tri-iodothyronine (T3) is a sensitive marker of endogenous hyperthyroidism. In levothyroxine (T4)-induced hyperthyroidism, there is no reason for T3 to be elevated, but this test is often requested in over-treated hypothyroid patients. This study investigated how informative T3 levels are in these patients. Our hypothesis is that T3 measurement would not add anything to the assessment of T4 over-replacement in primary hypothyroidism. Over a 15-week period, consecutive thyroid function test requests in patients on levothyroxine had T3 levels measured if thyroid-stimulating hormone (TSH) was below the reference range (RR; <0.27 miu/L) and free T4 was within or above the RR (12-22 pmol/L). Those with fully suppressed TSH (<0.02 mu/L) and high free T4 (>27 pmol/L) were defined as being over-replaced, while those with low, but measurable TSH and a normal free T4 were defined as unlikely to be over- replaced (control group). Receiver operating characteristic (ROC) curve analysis was used to assess the discriminant power of T3 to detect over-replacement. Of the 542 patients examined, 33 were included in the over-replaced group and 236 patients in the control group. A total of 273 patients were excluded for not fulfilling the criteria for either of these groups. In the over-replaced group, none had a raised T3. The most discriminant T3 level, using ROC curve analysis, was 1.6 nmol/L (RR=1.3-2.6 nmol/L), with a corresponding sensitivity and specificity of 58% and 71%, respectively (P=0.16). T3 levels bear little relation to thyroid status in patients on levothyroxine replacement, and normal levels can be seen in over-replaced patients. Measurement of T3 in this situation is of doubtful clinical value. WHAT'S ALREADY KNOWN ABOUT THIS TOPIC?: Thyroid function tests are the way that adequacy of levothyroxine replacement is determined. Where the test is available, T3 is often requested together with T4 and TSH by clinicians. The question is whether T3 measurement adds any further information. WHAT DOES THIS ARTICLE ADD?: The presented data supports the position that T3 measurement does not add anything to the interpretation of thyroid hormone levels in subjects with hypothyroidism on levothyroxine replacement therapy. Unnecessary testing could be avoided if this were more widely appreciated. In addition, over-replacement, with its attendant risks, would be more readily recognised and not wrongly excluded on the basis of a falsely reassuring normal T3 result.
三碘甲状腺原氨酸（T3）是内源性甲状腺功能亢进的敏感标志物。在左甲状腺素（T4）诱导的甲状腺功能亢进中，T3没有升高的理由，但在甲状腺功能减退治疗过度的患者中经常会要求进行此项检查。本研究调查了T3水平在这些患者中的信息量。我们的假设是，在原发性甲状腺功能减退中，T3测量对评估T4替代过量并无帮助。在15周的时间里，对接受左甲状腺素治疗的患者进行连续的甲状腺功能检查，如果促甲状腺激素（TSH）低于参考范围（RR；<0.27 mIU/L）且游离T4在RR范围内或高于RR（12 - 22 pmol/L），则测量T3水平。TSH完全被抑制（<0.02 μ/L）且游离T4高（>27 pmol/L）的患者被定义为替代过量，而TSH低但可测量且游离T4正常的患者被定义为不太可能替代过量（对照组）。采用受试者操作特征（ROC）曲线分析来评估T3检测替代过量的判别能力。在检查的542例患者中，33例被纳入替代过量组，236例被纳入对照组。共有273例患者因不符合这两组中的任何一组标准而被排除。在替代过量组中，没有人T3升高。使用ROC曲线分析，最具判别力的T3水平为1.6 nmol/L（RR = 1.3 - 2.6 nmol/L），相应的灵敏度和特异度分别为58%和71%（P = 0.16）。在接受左甲状腺素替代治疗的患者中，T3水平与甲状腺状态关系不大，替代过量的患者也可出现正常水平。在这种情况下测量T3的临床价值存疑。关于该主题已知的信息：甲状腺功能检查是确定左甲状腺素替代是否充分的方法。在可行此项检查的情况下，临床医生经常会同时要求检测T3、T4和TSH。问题在于T3测量是否能提供更多信息。本文补充了什么内容：所呈现的数据支持这样的观点，即在接受左甲状腺素替代治疗的甲状腺功能减退患者中，T3测量对甲状腺激素水平的解读并无帮助。如果能更广泛地认识到这一点，就可以避免不必要的检查。此外，替代过量及其伴随的风险将更容易被识别，而不会因T3结果正常而被错误排除，误以为结果令人放心。
