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Abstract
Background Asfotase alfa is the only approved treatment that can normalize mineralization in patients with hypophosphatasia (HPP). Its interference in alkaline phosphatase (ALP) dependent immunoassays has been reported. Objective To describe thyroid function tests interfered with by asfotase alfa and elucidate the underlying mechanism. Patients and Methods. Three patients with HPP treated with asfotase alfa were included. Thyroid hormone levels measured using five different immunoassays with or without ALP as a labeling enzyme during asfotase alfa treatment were evaluated. Results After the initiation of asfotase alfa, three HPP patients showed low free triiodothyronine (FT3) and free thyroxine (FT4) measured with AIA-2000 (Tosoh, Tokyo, Japan), an enzyme immunoassay system that uses ALP as a labeling enzyme, but their thyroid-stimulating hormone (TSH) levels were within the normal range. The other CLEIA system using ALP as a label, AIA-CL2400 (Tosoh, Tokyo, Japan), and ALP-independent immunoassay systems demonstrated normal FT3 and FT4 levels. These data suggested that although the thyroid function of these three patients was normal, asfotase alfa interfered with the thyroid hormone measurements made with AIA-2000. AIA-2000 and AIA-CL2400 adopted one-step and delayed one-step measurements, respectively, and the same antibody was used for both immunoassays. However, asfotase alfa may be absorbed on the magnetic beads used in the AIA reagent with the AIA-2000 system but not absorbed on the microparticles used in AIA-CL2400. Conclusion Clinicians should be aware of the possible interference in thyroid function measurements by adopting specific types of immunoassays in asfotase alfa-treated HPP patients.
Highlights
Hypophosphatasia (HPP) is an inherited disorder caused by loss-of-function mutations of the gene that encodes tissuenonspecific alkaline phosphatase (TNSALP), which is known as ALPL [1,2,3]
Measurement of FT3, FT4, and thyroid-stimulating hormone (TSH) Levels Using Various Immunoassay Systems. yroid-stimulating hormone (TSH), free triiodothyronine (FT3), and free thyroxine (FT4) in the three adult participants with HPP with or without asfotase alfa were measured in five immunoassay systems: AIA-2000 (Tosoh, Tokyo, Japan), AIA-CL2400 (Tosoh, Tokyo, Japan), AccuraSeed (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan), ARCHITECT i2000SR (Abbott Diagnostics, Chicago, Illinois), and Cobas 8000 e801 (Roche Diagnostics, Rotkruez, Switzerland) (Table 1)
A thyroid function test using AIA2000, an immunoassay system that uses ALP as a labeling enzyme, revealed abnormally low levels of FT3 and FT4 despite normal TSH levels in three patients with HPP treated with asfotase alfa
Summary
Hypophosphatasia (HPP) is an inherited disorder caused by loss-of-function mutations of the gene that encodes tissuenonspecific alkaline phosphatase (TNSALP), which is known as ALPL [1,2,3]. Yroid hormone levels measured using five different immunoassays with or without ALP as a labeling enzyme during asfotase alfa treatment were evaluated. After the initiation of asfotase alfa, three HPP patients showed low free triiodothyronine (FT3) and free thyroxine (FT4) measured with AIA-2000 (Tosoh, Tokyo, Japan), an enzyme immunoassay system that uses ALP as a labeling enzyme, but their thyroid-stimulating hormone (TSH) levels were within the normal range. Ese data suggested that the thyroid function of these three patients was normal, asfotase alfa interfered with the thyroid hormone measurements made with AIA-2000. Clinicians should be aware of the possible interference in thyroid function measurements by adopting specific types of immunoassays in asfotase alfa-treated HPP patients
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