Depression is very common in older people and can affect up to 13% of this population. Some scientists have suggested that low levels of vitamin D can cause depression. Low levels of vitamin D can also cause an increase in parathyroid hormone levels which are in turn frequently accompanied by an increase in depressive symptoms. In fact people's mood usually returns to normal after treatment of hyperparathyroidism. A Dutch study of 1,282 elderly people between the ages of 65-95 found that levels of vitamin D were 14% lower in 169 people with minor depression and 14% lower in 26 people with major depression. Levels of parathyroid hormone were 5% and 33% higher respectively. Depression severity was significantly associated with decreased vitamin D and increased parathyroid hormone levels.
Hoogendijk, Witte J.G. ... [et al] - Depression is associated with decreased 25-hydroxyvitamin D and increased parathyroid hormone levels in older adults Archives of General Psychiatry May 2008, 65(5), 508-512