…that low vitamin D levels impact Serotonin receptors in a negative way and can lead to depression and/or anxiety in patients with this deficiency?

This is why depression is often seen by patients living in states/countries/territories in the northern latitudes where some days only have a few hours of daylight during the winter months of the year. “Sun lamps” have been used to help many people who suffer from this condition of seasonal depression; also called seasonal affective disorder or SAD. The best way to determine adequate vitamin D levels is by measuring 25-OH vitamin D levels in blood.