The study analyzes the influence of sodium hydroxide, caustic soda, or NaOH on the elasticity and elongation of spandex as a constituent fiber in stretch denim fabrics. The textile substrate under analysis has a composition of 70% cotton, 19% polyester, 8% viscose, and 3% spandex. The weight per unit area of the denim fabric is 341 g/m2, classified as a heavyweight fabric. To analyze the behavior of caustic soda on the fabric, several recipes for shade lightening were developed, modifying the amount of sodium hydroxide to 1%, 2%, 3%, 4%, and 5%, like those used in denim processing plants. The test variable was the amount of caustic soda in the post-treatment of the dyed fabric as an auxiliary in tone reduction recipes. Finally, the ISO 13934–1 textile test was applied using a Titan 5 dynamometer to establish the degree of fabric impact in the warp or longitudinal direction and the weft or transverse direction. The findings indicate a direct impact on the elasticity and elongation of the fabric. Scientific evidence suggests that the dosage of caustic soda significantly affects the weft threads, as the impact on elasticity decreases by 30.69%.