Altered Thyroid Function Tests Observed in Hypophosphatasia Patients Treated with Asfotase Alfa

Area of Science:

• Biochemistry
• Endocrinology
• Clinical Diagnostics

Background:

• Asfotase alfa is the sole approved treatment for hypophosphatasia (HPP), normalizing mineralization.
• Previous reports indicate asfotase alfa interferes with alkaline phosphatase (ALP)-dependent immunoassays.

Purpose of the Study:

• To identify specific thyroid function tests affected by asfotase alfa.
• To elucidate the mechanism behind this interference.

Main Methods:

• Evaluated thyroid hormone levels (FT3, FT4, TSH) in three HPP patients on asfotase alfa.
• Utilized five different immunoassays, including ALP-dependent and independent systems.

Main Results:

• Falsely low free triiodothyronine (FT3) and free thyroxine (FT4) were observed with the AIA-2000 immunoassay (using ALP).
• TSH levels remained normal, and other ALP-independent assays showed accurate FT3 and FT4.
• Interference is attributed to asfotase alfa binding to magnetic beads in the AIA-2000 system.

Conclusions:

• Asfotase alfa can interfere with specific thyroid function tests, particularly those using ALP and magnetic bead-based immunoassays.
• Clinicians must be aware of potential assay interference in HPP patients undergoing asfotase alfa treatment.