Section 862.1700 - Total thyroxine test system(a)Identification. A total thyroxine test system is a device intended to measure total (free and protein bound) thyroxine (thyroid hormone) in serum and plasma. Measurements obtained by this device are used in the diagnosis and treatment of thyroid diseases.(b)Classification. Class II (special controls). The device is exempt from the premarket notification procedures in subpart E of part 807 of this chapter subject to the limitations in § 862.9 .52 FR 16122, May 1, 1987, as amended at 84 FR 71797 , Dec. 30, 2019