"Adolescents within tier 4 adolescent mental health services may be at higher risk of vitamin D deficiency and so assessment of vitamin D levels should be considered as part of a standard physical health review for this group of young people."
So said the results reported by Neil Stewart & Simon Lewis  (open-access) who surveyed blood test results of patients admitted to a tier 4 psychiatric unit for vitamin D levels. Such a study was conducted on the basis that "it is plausible that vitamin D and/or vitamin D deficiency have a role in the pathogenesis of mental illness." Yes, indeed I might agree (see here and see here for examples).
Authors identified some 27 individuals who were tested for vitamin D deficiency between 2012 and 2014 from a population of 56. Over 80% (22/27) had vitamin D levels falling into the deficient or severely deficient range, and none of them had vitamin D levels reaching the bottom end of the typical range (75–250 nmol/L). A few other points are worthwhile noting: "In individuals from BME [black and minority ethnic] groups, who were potentially at higher risk of vitamin D deficiency due to increased skin pigmentation, 52.9% (9/17) were tested for vitamin D levels and 100% were deficient or severely deficient."
I was rather happy to see that authors have very much stuck to their findings minus too much speculation about their meaning. They, for example, suggest that all patients entering their particular service should "be considered at high risk of vitamin D deficiency" for whatever reason(s). They emphasise that vitamin D testing should be part and parcel of the routine physical examination normally provided to patients. They even talk about correcting any deficiency/insufficiency whilst monitoring vitamin D levels for any adverse effects or toxicity. In short, treat the physical health of their patients/service users despite the focus of their service being psychiatric. Lessons I'm sure that could be applied to many different labels/diagnoses with a behavioural or psychiatric element to them.
Going back to the idea that vitamin D deficiency might play a role in various conditions/states outside of those linked to bone health, the authors add to other voices suggesting that more investigation is needed to confirm/refute links between vitamin D status and behavioural or psychiatric issues. They note: "If an association between depression and vitamin D deficiency were to be confirmed through future study, vitamin D supplementation could potentially be a cost-effective treatment adjunct with minimal adverse effects." Again, I can't argue with the logic.
 Stewart NF. & Lewis SN. Vitamin D deficiency in adolescents in a tier 4 psychiatric unit. BJPsych Bull. 2017 Jun;41(3):133-136.