I wrote 2 years
ago on the seasonal variations of plasma normetanephrine levels in patients
without pheo. In early 2014, a paper reported that the normetanephrine levels are
20% higher in wintertime than in summertime in the Netherlands and Germany, and
a second study showed that normetanephrine levels are 40% higher in wintertime
than in summertime in Los Angeles, USA. As the Netherlands and Germany have
temperate climate and Los Angeles has Mediterranean climate, higher
normetanephrine levels in wintertime seem to be pretty universal. At that time,
I suggested a similar study in a tropical area like Hawaii with minimal
temperature differences throughout the year. One would predict that the plasma
normetanephrine levels in people without pheo remain unchanged throughout the
year in Hawaii.
A third study is
indeed done, although not in Hawaii, and published recently. It is done in the
West of Ireland. Ireland has a temperate oceanic climate. The average
summertime and wintertime temperatures are 14.3 and 5.8 °C respectively in the
West of Ireland. The temperature difference of 8.4 °C in the West of Ireland is
very similar to that in Los Angeles (8.6 °C), although it is generally much cooler
in the West of Ireland (average Los Angeles summertime and wintertime
temperatures are 23.2 and 14.6 °C respectively). The seasonal temperature
differences are much smaller in the West of Ireland and Los Angeles than those
in the Netherlands and Germany (17 °C). Unlike in Los Angeles, there is,
however, no difference in the plasma normetanephrine levels between the
summertime and wintertime in the West of Ireland.
There could be
multiple potential explanations of the different result obtained from the third
study. An obvious issue is that all three studies are retrospective ones which
can lead to certain biases. The most important lesson, however, is that medical
studies need to be reproduced in different settings. We cannot assume that results
from a previous study should be readily applied to another setting.
Dr. Pheo
