Table 1. The features and outcomes of research articles included in the systematic review.

LH, luteinizing hormone; FSH, follicle-stimulating hormone; TT, total testosterone; FAI, free androgen index; FT, free testosterone.

Author	Study title	Study type	Country of study	Age of the study population	Sample size	Study results
Lerchbaum, E et al. [1]	Effects of vitamin D supplementation on androgens in men with low testosterone levels: a randomized controlled trial	RCT	Austria	≥18	100	Participants received either a placebo or 20,000 IU of oral vitamin D weekly for 12 weeks. No significant differences were observed in testosterone levels or other endocrine parameters between the intervention and placebo groups.
Amini L et al. [23]	Evaluation of the effect of vitamin D3 supplementation on quantitative and qualitative parameters of spermograms and hormones in infertile men: A randomized controlled trial	RCT	Iran	35-39	62	Participants received either a placebo or 50,000 IU of Vitamin D3 once a week for eight weeks. No significant differences were observed in spermogram parameters, LH, FSH, TT, and FAI levels between the two groups. However, the intervention group had decreased SHBG levels (p = 0.01), while the placebo group showed increased FT levels (p = 0.03).
Chi Chen et al. [24]	Causal link between vitamin D and total testosterone in men: a Mendelian randomization analysis	MRA	China	≥18 years old	4,254	The study revealed that lower vitamin D levels (25(OH)D) were linked to reduced total testosterone (T) levels. Genetic risk for lower vitamin D (VD_GRS) was also associated with decreased 25(OH)D and total T levels. Mendelian randomization analysis has indicated a potential causal relationship, wherein elevated 25(OH)D levels are associated with a 0.12 increase in total T. Adequate vitamin D is essential for proper testosterone regulation.
Kok-Yong Chin et al. [9]	Vitamin D is significantly associated with total testosterone and sex hormone-binding globulin in Malaysian men	Cross-sectional	Malaysia	20 or above	382	Individuals with vitamin D deficiency (25-hydroxyvitamin D levels below 50 nmol/L) exhibited notably reduced levels of total testosterone and sex hormone-binding globulin (SHBG) in comparison to those with vitamin D insufficiency and optimal levels (statistically significant with p <0.05). However, these differences became statistically nonsignificant after adjusting for body mass index (BMI) (p > 0.05).
Lerchbaum E et.al. [22]	Vitamin D and testosterone in healthy men: a randomized controlled trial	RCT	Austria	≥18	98	In healthy men with normal 25-hydroxyvitamin D and total testosterone levels, vitamin D treatment did not significantly affect total testosterone. However, it had a significant impact on insulin sensitivity (QUICKI) and showed a trend toward decreasing the Matsuda index
Sim MY et al. [25]	Seasonal variations and correlations between vitamin D and total testosterone levels	Cross-sectional	Korea	25-86	1,559	There was no significant association between serum 25-hydroxyvitamin D (25(OH)D) and total testosterone levels in Korean men. However, both 25(OH)D and total testosterone levels showed significant seasonal variations that remained significant even after adjusting for confounding variables.
Wentz LM et al. [27]	Vitamin D correlation with testosterone concentration in male US soldiers and veterans	Cross-sectional	USA	>25	796	The average serum vitamin D level was 29.2 ± 11.1 ng/mL, with 55.7% having deficient or insufficient levels. Lower vitamin D levels in the lowest quintile were associated with lower testosterone levels, younger age, and higher BMI. However, when accounting for BMI, age, and timing of testosterone measurement, the significance of vitamin D in predicting testosterone levels disappeared.
Rafiq R et al. [26]	Associations of vitamin D status and vitamin D-related polymorphisms with sex hormones in older men	Cross-sectional	Dutch	65-89 years	643	Lower serum 25-hydroxyvitamin D (25(OH)D) levels were associated with reduced total and bioavailable testosterone. Men with 25(OH)D levels below 25 nmol/L, 25-50 nmol/L, and 50-75 nmol/L had lower total testosterone compared to those above 75 nmol/L after accounting for confounding factors. The association between 25(OH)D and bioavailable testosterone was significant only for men with levels below 25 nmol/L. No significant relationships were found with SHBG, estradiol, or gonadotropin levels. Hypogonadism was not linked to lower 25(OH)D levels. Gene polymorphisms related to vitamin D did not significantly affect hormone levels or alter associations between 25(OH)D and sex hormones or gonadotropins.