Test ID: FRT4D
T4 (Thyroxine), Free, Dialysis, Serum

Determining thyroid status of sick, hospitalized patients

确定已经鉴定出异常结合蛋白的患者的甲状腺状态

Possibly useful in pediatric patients

Thyroxine (T4) and triiodothyronine (T3) are the 2 biologically active thyroid hormones. T4 makes up more than 80% of circulating thyroid hormones.

甲状腺分泌后，大约70％的循环T4和T3与甲状腺结合球蛋白（TBG）结合，而每个循环中的T4和T3分别分别与transthyretin（TTR）和白蛋白结合了10％至20％。小于0.1％的自由T4（FT4）或游离T3（FT3）循环。FT4和FT3通过扩散自由输入和离开细胞。只有游离激素具有生物活性，但结合和游离的分数处于平衡状态。TTR和白蛋白的平衡很快。相比之下，TBG结合甲状腺激素非常紧密，平衡解离慢。从生物学上讲，结合TBG的甲状腺激素是激素储层，T4用作T3的激素。在细胞内，T4要么转换为T3，其有效的5倍，或反向T3，这是生物学上无活性的T3。最终，T3和T4少得多，与核甲状腺激素受体结合，以组织特异性方式改变基因表达模式。

Under normal physiologic conditions, FT4 and FT3 exert direct and indirect negative feedback on pituitary thyrotropin (thyroid-stimulating hormone: TSH) levels, the major hormone regulating thyroid gland activity. This results in tight regulation of thyroid hormone production and constant levels of FT4 and FT3 independent of the binding protein concentration. Measurement of FT4 and FT3, in conjunction with TSH measurement, therefore represents the best method to determine thyroid function status. It also allows determination of whether hyperthyroidism (increased FT4) or hypothyroidism (low FT4) are primary (the majority of cases, TSH altered in the opposite direction as FT4) or secondary/tertiary (hypothalamic/pituitary origin, TSH altered in the same direction as FT4). By contrast, total T4 and T3 levels can vary widely as a response to changes in binding protein levels, without any change in free thyroid hormone levels and, hence, actual thyroid function status.

FT4 is usually measured by automated analog immunoassays. In most instances, this will result in accurate results. However, abnormal types or quantities of binding proteins found in some patients and most often related to other illnesses or drug treatments, may interfere in the accurate measurement of FT4 by analog immunoassays. These problems can be overcome by measuring FT4 by equilibrium dialysis, free from interfering proteins.

0.8-2.0 ng/dl

参考值适用于所有年龄段。

所有游离激素测定应与甲状腺激素（甲状腺刺激激素）测量结合。

Free thyroxine (FT4) levels below 0.8 ng/dL indicate possible hypothyroidism. FT4 levels above 2.0 ng/dL indicates possible hyperthyroidism.

Neonates can have significantly higher FT4 levels. The hypothalamic-pituitary-thyroid axis can take several days or, sometimes, weeks to mature.

Certain drugs may cause short-term free thyroxine fluctuations.

-肝素

-Salicylates

- Acetylsalicylic acid (aspirin)

- Salicylic acid (salsalate)

-Furosemide

-Fenclofenac

-Mefenamic acid

-Flufenamic acid

-Diclofenac

-Diflunisal

-Phenytoin

-Carbamazepine

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4. Soldin SJ, Soukhova N, Janici N, Jonklaas J, Soldin OP: The measurement of free thyroxine by isotope dilution tandem mass spectrometry. Clin Chimica Acta. 2005;358:113-118

5. Sakai H，Nagao H，Sakurai M等：通过液相色谱串联质谱法和免疫测定法测量的3,3'，5'-Triioodothyronine和甲状腺激素之间的血清水平与甲状腺激素之间的相关性。PLOS ONE。2015; 10（10）：e0138864 doi：10.1371/journal.pone.0138864;校正：PLOS ONE。2016; 11（7）：E0159169

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