Summer vs Winter Blood Tests: Seasonal Changes

Do seasons affect blood test results?

Yes -- and this is not a myth but a scientifically confirmed phenomenon. Many blood markers naturally fluctuate depending on the season. These changes are physiological -- they reflect the body's adaptation to environmental conditions -- but they can be misleading if you are not aware of them.

For example, winter TSH may look like mild hypothyroidism when it is actually a normal seasonal fluctuation. Or a vitamin D level checked in summer may appear normal even though in winter it drops below the normal range.

Understanding these fluctuations is important for two reasons: first, so you do not panic over normal changes, and second, so you know when it is most appropriate to test and how to correctly compare results from different seasons.

Vitamin D -- the greatest seasonal variability

Vitamin D has the most pronounced seasonal fluctuation of all blood markers. Its primary source is UVB sunlight, which stimulates vitamin D production in the skin. At Lithuanian latitudes (54-56 degrees north), from October to March the sun angle is too low for UVB rays to reach the earth's surface with sufficient intensity -- vitamin D production in the skin virtually stops during this time.

Studies show that up to 48% of Europeans have vitamin D deficiency (25(OH)D < 20 ng/mL or < 50 nmol/L) during winter months. In summer, this number drops to 10-15% (Source -- Cashman, 2009). Lithuania, being one of the northernmost EU countries, falls in the highest risk zone.

The difference between the summer peak and winter low can reach 15-25 ng/mL -- a significant fluctuation that can lead to different interpretations. Therefore, the most valuable time to test vitamin D is in spring (March-April), when levels are at their lowest -- this allows an objective assessment of the true deficit and whether supplementation is needed.

If you check vitamin D in summer and get a normal result, this does not mean it will also be within range in winter. This seasonal context is very important.

TSH -- thyroid seasonal rhythm

TSH naturally increases in winter and decreases in summer. The difference can reach 0.3-0.5 mIU/L -- a significant amount given that TSH reference ranges are narrow (typically 0.4-4.0 mIU/L) (Source -- Kim, 2013).

This fluctuation is related to the body's thermoregulation and daylight duration. In winter, the body slightly increases its need for thyroid hormones to maintain body temperature and energy metabolism during the dark season. The pituitary responds by increasing TSH secretion.

The practical significance is important: if your TSH is 4.2 mIU/L in winter (slightly above the formal upper limit), this may be a normal seasonal increase rather than true subclinical hypothyroidism. Before starting treatment, a doctor may recommend repeating the test in summer to assess whether TSH returns to normal on its own.

Of course, if TSH is significantly elevated (above 7-10 mIU/L) at any time of year, seasonal fluctuation does not explain it and further evaluation is needed.

Hemoglobin and hematocrit

Hemoglobin and hematocrit are usually slightly higher in winter than in summer. This is related to several physiological mechanisms. In winter, the body experiences less perspiration (sweating), and cold causes peripheral vasoconstriction -- blood vessels narrow. As a result, blood plasma volume slightly decreases and blood becomes more concentrated -- this is called hemoconcentration (Source -- Thirup, 2008).

The difference is not large -- typically 2-5 g/L for hemoglobin -- but can matter if the reading is near the reference range boundary. In winter, hemoglobin may appear normal when in summer it would be below normal and reveal developing anemia.

Also, in summer, due to greater fluid intake and sweating, blood can be slightly more diluted -- this is called hemodilution. For this reason, summer hemoglobin may appear slightly lower than it actually is.

CRP and inflammation markers

hs-CRP may be higher in winter for several related reasons: more frequent colds and upper respiratory tract infections (even mild forms raise CRP), less physical activity during dark months, lower vitamin D (which has anti-inflammatory effects), and dietary changes in winter -- more processed, calorie-dense food and fewer fresh fruits and vegetables (Source -- Rudnicka, 1997).

Therefore, CRP taken in January is best compared with CRP taken at the same time the previous year, rather than with a summer result. Seasonal comparison is more informative.

Ferritin and other markers

Ferritin can fluctuate due to indirect seasonal factors -- frequency of infections, dietary changes, and physical activity levels. Since ferritin is also an acute-phase protein (it rises during inflammation), its increase in winter may be related to the inflammatory response rather than iron excess, because we get sick more often in winter.

Other markers that may fluctuate slightly by season: cholesterol (typically higher in winter), platelets, and white blood cells. These fluctuations are usually small and clinically insignificant in healthy individuals.

Practical plan

1. Schedule baseline tests at the same time of year annually -- this way you compare similar conditions and see real changes rather than seasonal fluctuations. 2. The best time for most baseline tests: late May or early September -- seasonal fluctuations are smallest during these periods. 3. Check vitamin D in spring (March-April) -- you will see the true winter deficit and can plan supplementation. 4. If TSH rises in winter and is only slightly above normal -- discuss with your doctor repeating the test in summer before making treatment decisions. 5. When comparing results from different seasons, always consider the time of year -- this helps avoid incorrect interpretations and unnecessary worry.

When to act immediately

Seasonal fluctuations are small and subtle. If markers deviate significantly from normal at any time of year -- it is certainly not the season's fault, and medical evaluation is needed. See your doctor if: vitamin D is below 10 ng/mL (severe deficiency), TSH is above 10 mIU/L or below 0.1 mIU/L, hemoglobin is below 100 g/L (significant anemia), or hs-CRP is above 10 mg/L (possible acute inflammation).

This is not medical advice. Consult your doctor for health decisions.