Low vitamin D is common, especially in winter, and is linked to lower testosterone in observational studies. What the trials show, sensible doses and upper limits.
Low vitamin D is common and has been linked to lower testosterone in observational studies. Trial results are mixed: a small 2011 trial (Pilz et al.) found that men taking vitamin D3 for a year had a rise in testosterone compared with placebo, but most of them started out deficient, and trials in men whose levels were normal to begin with have generally not found a benefit. If you're low, correcting it is worthwhile for its own sake, and a blood test will tell you.
Vitamin D receptors (VDRs) are present on Leydig cells — the testicular cells responsible for testosterone production. This suggests vitamin D may play a role in testosterone production. Several observational studies have found that men with higher blood vitamin D tend to have higher testosterone, but studies like these can't show cause and effect. In the Pilz trial, men in a weight-loss program, most of them vitamin D deficient, took 3,332 IU of vitamin D3 a day or placebo for a year; total testosterone rose in the vitamin D group but not the placebo group.
Your skin makes vitamin D from UVB sunlight. At northern latitudes, including much of the northern US, winter sunlight is too weak for much vitamin D production, so levels often fall over the winter. People with darker skin, those who spend little time outdoors, older adults and people with obesity are also more likely to be low. The NIH recommends 600 IU (15 mcg) a day for adults up to age 70 and 800 IU over 70, with an upper limit of 4,000 IU (100 mcg) a day. If a blood test shows you're deficient, your doctor may recommend a higher dose for a period.
Testosil provides 4,000 IU of vitamin D3 per serving, which already reaches the adult upper limit; two servings, or an added multivitamin, would exceed it. TestRX lists 1,140 IU per serving on its published label (we couldn't confirm this on the official site). Neither product has been tested in its own clinical trial, so any benefit from the vitamin D is most likely if you were low to begin with.