SPINA (Structure Parameter Inference Approach) is a method to calculate constant structure parameters of endocrine feedback control systems from serum or plasma hormone levels obtained in vivo. Equations are derived from mathematical models of the respective information processing structures. First implementations cover thyroid homeostasis and carbohydrate metabolism. Application of SPINA on the assessment of thyrotropic feedback control is increasingly used e.g. in experimental thyroid research. Calculations are supported by software tools. See also: GD, GT