Vitamin D plays a well-established role in immune system regulation, and the thymus gland is a central organ in the development of the adaptive immune system, particularly T-cell maturation. Scientific studies have shown that vitamin D receptors are present in thymic tissue, and vitamin D influences thymocyte (immature T-cell) proliferation and differentiation. Several in vitro and animal studies demonstrate that vitamin D deficiency can lead to thymic atrophy and impaired T-cell function, suggesting its importance in thymic health.

In humans, observational studies connect vitamin D deficiency with altered immune function and increased susceptibility to infections, though direct clinical trials examining vitamin D supplementation specifically for "thymus support" are limited. Most research focuses on broader immune outcomes rather than direct measurement of thymic structure or function. Nevertheless, the mechanistic understanding and indirect evidence provide moderate scientific validation for vitamin D’s role in supporting the thymus and immune system.

In summary, while there is significant mechanistic and preclinical evidence, and vitamin D is important for immune health, direct clinical evidence for vitamin D supplementation explicitly to support thymus function in healthy adults is limited, justifying a moderate evidence rating.