Cod liver oil is primarily valued for its high content of vitamins A and D, as well as omega-3 fatty acids (EPA and DHA). These nutrients play significant roles in various physiological processes, including those related to the glandular (endocrine) system. Vitamin D is critical for calcium metabolism and bone health, but it also modulates immune and endocrine function, including supporting thyroid and pancreatic health. Vitamin A is essential for cellular differentiation and immune function, and deficiency can negatively affect glandular tissues such as the adrenal and thyroid glands.

Scientific studies support the role of vitamin D in modulating the endocrine system, particularly in relation to thyroid function and insulin sensitivity. However, direct evidence specifically linking cod liver oil supplementation to broad "glandular system" support is limited. Most studies focus on its benefit in correcting vitamin D deficiency, immune health, and reducing inflammation. The omega-3 fatty acids in cod liver oil have also been shown to have anti-inflammatory effects, which can indirectly benefit the function of endocrine glands.

In summary, while there is scientific rationale for using cod liver oil to support certain aspects of glandular health due to its nutrient content, comprehensive clinical evidence for its use as a general “glandular system” tonic is moderate. The evidence is strongest for its vitamin D and A content, but less so for direct, broad endocrine benefits.