Month of birth and thymic output.

I t is widely acknowledged that month of birth influences the risk for developing multiple sclerosis (MS) later in life. This has been consistently shown in a number of countries and regions including England, Scotland, Canada, Denmark, Sweden, Sardinia, Finland, and Australia (eReferences, http://www.jamaneuro.com). The month of birth effect is particularly evident in England, where the risk for MS peaks in May and drops in November for those born in those months. The biological basis of this month of birth effect is not yet known. Because thymic development mainly occurs in utero and is susceptible to intrauterine exposures, we sought to assess whether the month of birth has any effect on thymic function. Given the evidence accumulating for vitamin D deficiency in the etiology of MS, we also investigated whether vitamin D levels were associated with thymic output.