Positive association between vitamin D and testosterone levels have been observed from various research studies. Randomised controlled trials (RTA) with 1,362 participants, conducted by Nimptsch et al (2013) has concluded a positive association of vitamin D with total and free testosterone levels. The association is linear at lower levels of vitamin D, reaching a plateau at high levels.
In other words, men with low vitamin D are likely to be found suffering from testosterone deficiency. In the UK, we might not get adequate sunshine throughout the whole year but Vitamin D can be found in milk, egg yolks, cheese, cod liver oil and so on. Supplementation of vitamin D is required when the level is found to be insufficient or deficient.