Vitamin D and Testosterone: Evidence, Dosing, and AU Context

Vitamin D (specifically its active form, 1,25-dihydroxyvitamin D3 or calcitriol) acts as a steroid hormone — it binds to nuclear receptors throughout the body. Vitamin D receptors are present on Leydig cells (the testosterone-producing cells), Sertoli cells, and in the hypothalamus and pituitary. Animal models show that vitamin D receptor knockout mice develop hypogonadism, confirming a functional role in testosterone biosynthesis.

The proposed mechanism: vitamin D upregulates the expression of steroidogenic enzymes in Leydig cells, including CYP11A1 (which converts cholesterol to pregnenolone, the first step in testosterone synthesis) and reduces expression of aromatase (CYP19A1), the enzyme that converts testosterone to oestradiol. This dual action would support testosterone and reduce its conversion to oestrogen.

Wehr et al. (2010) found in 2,299 men that 25-hydroxyvitamin D levels positively correlated with total testosterone and free testosterone across the entire range, with the strongest association in men who were overtly deficient (25-OH-D below 50 nmol/L). Men with sufficient vitamin D (>75 nmol/L) had testosterone levels significantly higher than deficient counterparts.

Seasonal variation data supports this: testosterone tracks vitamin D in population studies, with both peaking in summer and falling in late winter. In Melbourne, for example, 25-OH-D in office workers averages 45–55 nmol/L in winter — below the 75 nmol/L sufficiency threshold — while Sydney workers fare somewhat better due to latitude and sunlight hours.

Pilz et al. (2011) conducted a year-long RCT in overweight men, finding that 3,332 IU/day of vitamin D3 significantly raised testosterone compared to placebo (from 10.7 to 13.4 nmol/L — a 25% increase) while placebo showed no change. This is frequently cited as the definitive vitamin D + testosterone study.

However, subsequent trials in men without confirmed deficiency have been less impressive. Canguven et al. (2017) found positive effects; Guo et al.'s meta-analysis (2017) found significant effects only in deficient populations. The pattern is the same as zinc: repletion works when you're genuinely deficient; supplementation above sufficiency shows diminishing returns.

Despite sun exposure, vitamin D deficiency affects 23–30% of Australian adults (AIHW, 2022), with rates higher in: men working office jobs in southern states (Melbourne, Hobart, Adelaide) between April and September; dark-skinned men who require longer UV exposure for equivalent synthesis; men using high-SPF sunscreen year-round; and shift workers or men with limited outdoor activity.

Australian Institute of Health and Welfare data shows that Bali-based Australians and frequent Bali expatriates, conversely, typically have adequate vitamin D year-round due to equatorial sun exposure — but may have other hormone-disrupting factors (alcohol, chronic stress, dietary changes) that interact with the vitamin D picture.

The target for most men is 25-OH-D of 75–120 nmol/L. To achieve this from deficiency, 3,000–5,000 IU/day of vitamin D3 (cholecalciferol) for 8–12 weeks is standard, followed by maintenance at 1,000–2,000 IU/day or year-round moderate sun exposure (10–20 minutes of midday sun on arms and legs, without sunscreen, 3–5 times per week). Vitamin D3 is more effective than D2 for raising serum levels.

Dr. Nikola Topalovic, MD PhD reviews every FORM client report and routinely interprets vitamin D alongside testosterone, SHBG, and sensitive estradiol — because low vitamin D in the context of low total T and elevated SHBG is a common and correctable pattern in Australian indoor-working men.