GD, also referred to as SPINA-GD, is an abbreviation for "gain of deiodinases". It is a mathematical symbol for calculated sum activity of peripheral 5' deionidases type 1 (DIO1) and type 2 (DIO2). Calculation of GD is part of a structure-parameter inference approach, a method that is used e.g. in experimental thyroid research. The equations are derived from a mathematical model of thyrotropic feedback control.