Jansen Heleen I, Bult Marijn M, Bisschop Peter H, Boelen Anita, Heijboer Annemieke C, Hillebrand Jacquelien J
Amsterdam UMC location Vrije Universiteit Amsterdam, Department of Clinical Chemistry, Endocrine Laboratory, Amsterdam, The Netherlands.
Amsterdam Gastroenterology, Endocrinology & Metabolism, Amsterdam, The Netherlands.
Endocr Connect. 2023 Mar 20;12(4). doi: 10.1530/EC-22-0538. Print 2023 Apr 1.
In our hospital, physicians noticed high free thyroxine (fT4) concentrations without complete suppression of thyroid-stimulating hormone (TSH) in blood samples of patients at the outpatient clinic, which appeared to occur more often following the introduction of a new fT4 immunoassay. This discordance may be explained by incorrect reference intervals, analytical issues, or patient-related factors. We aimed to establish the contribution of the possible factors involved.
Reference intervals of both fT4 immunoassays were re-evaluated using blood samples of healthy volunteers and the new immunoassay's performance was assessed using internal quality controls and external quality rounds. The frequency of discordant fT4 and TSH pairings obtained from laboratory requests were retrospectively analysed using a Delfia (n = 3174) and Cobas cohort (n = 3408). Last, a literature search assessed whether the time of blood draw and the time of levothyroxine (L-T4) ingestion may contribute to higher fT4 concentrations in L-T4 users.
The original reference intervals of both fT4 immunoassays were confirmed and no evidence for analytical problems was found. The Delfia (n = 176, 5.5%) and Cobas cohorts (n = 295, 8.7%) showed comparable frequencies of discordance. Interestingly, 72-81% of the discordant results belonged to L-T4 users. Literature indicated the time of blood withdrawal of L-T4 users and, therefore, the time of L-T4 intake as possible explanations.
High fT4 without suppressed TSH concentrations can mainly be explained by L-T4 intake. Physicians and laboratory specialists should be aware of this phenomenon to avoid questioning the assay's performance or unnecessarily adapting the L-T4 dose in patients.
在我们医院，门诊患者的血液样本中，医生注意到游离甲状腺素（fT4）浓度较高，而促甲状腺激素（TSH）未被完全抑制，这种情况在引入新的fT4免疫测定法后似乎更常出现。这种不一致可能是由于参考区间不正确、分析问题或患者相关因素导致的。我们旨在确定可能涉及的因素的作用。
使用健康志愿者的血液样本重新评估两种fT4免疫测定法的参考区间，并使用内部质量控制和外部质量评估来评估新免疫测定法的性能。对从实验室申请中获得的fT4和TSH配对不一致的频率进行回顾性分析，使用了Delfia队列（n = 3174）和Cobas队列（n = 3408）。最后，进行文献检索，评估采血时间和左甲状腺素（L-T4）摄入时间是否可能导致L-T4使用者的fT4浓度升高。
两种fT4免疫测定法的原始参考区间得到确认，未发现分析问题的证据。Delfia队列（n = 176，5.5%）和Cobas队列（n = 295，8.7%）显示出相当的不一致频率。有趣的是，72 - 81%的不一致结果属于L-T4使用者。文献表明，L-T4使用者的采血时间以及因此L-T4的摄入时间可能是解释原因。
fT4高而TSH浓度未被抑制主要可由L-T4摄入来解释。医生和实验室专家应意识到这一现象，以避免质疑检测方法的性能或不必要地调整患者的L-T4剂量。
