Vitamin B12, Vitamin D, and Thyroid Hormones: The Blood Markers Behind Brain Fog

If you’re feeling mentally foggy or fatigued, your blood tests might reveal a nutritional or hormonal cause. Low Vitamin B12 can cause memory issues, slow thinking, and cognitive fog. B12 is vital for nerve health and cognitive processing, and deficiencies — especially common in vegetarians, older adults, or people with gut absorption issues — are often reversible with supplementation.

Vitamin D plays a critical role in mood and cognitive function. Several studies link vitamin D deficiency to depressive symptoms, slow cognition, and lack of clarity. Boosting low D levels has been associated with improvements in both mental and physical energy.

Your thyroid also plays a central role. An underactive thyroid (elevated TSH, low FT4/FT3) slows down metabolism, leading to forgetfulness, poor concentration, and overall sluggishness.

Takeaways:

• Test B12, Vitamin D, TSH, and FT4/FT3 if you're experiencing persistent brain fog.
• Improve B12 through meat, dairy, eggs, or supplements — especially if plant-based.
• Correct Vitamin D with sun exposure, fatty fish, eggs, or supplementation.
• Manage thyroid health via prescribed treatment and ensure adequate iodine intake.
• Regular movement, stress management, and quality sleep further enhance cognitive clarity.

Test for these Biomarkers: Vitamin B12, Vitamin D, TSH, FT4, FT3

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