Deficiency symptoms are real, documented, and mostly non-specific — which is the part that makes self-diagnosis unreliable. Several very different deficiencies produce overlapping complaints, and the most common symptom of all, fatigue, is also the symptom with the longest list of unrelated causes.
What each authority actually lists
Vitamin B12. NIH ODS lists fatigue, megaloblastic anemia, glossitis of the tongue, palpitations, pale skin, weight loss, and neurological changes such as numbness and tingling in the hands and feet. It also notes that symptoms "can take several years to appear."
Vitamin C. NIH ODS states that insufficient intake causes scurvy, characterized by fatigue, widespread connective tissue weakness, and capillary fragility.
Vitamin A. NIH ODS describes xerophthalmia as the most common clinical sign, with night blindness — difficulty seeing in low light — as the first sign.
Iron. NIH ODS notes that iron deficiency affects gastrointestinal function, immune function, and body temperature regulation, and is "not uncommon" in young children, women of reproductive age, and pregnant women.
The overlap problem
Notice how many lists begin with fatigue. B12 deficiency, iron deficiency, and vitamin C deficiency all produce it. So do thyroid disorders, sleep debt, depression, and ordinary overwork. A symptom shared by that many conditions cannot, on its own, point to a nutrient.
The symptoms that carry more information are the specific ones: night blindness points toward vitamin A in a way fatigue points nowhere. Numbness and tingling point toward B12 more usefully than tiredness does.
Why the B12 pattern deserves separate attention
NIH ODS makes a point about B12 that applies to no other nutrient on this list: neurological symptoms "can occur without anemia, so early diagnosis and intervention is important to avoid irreversible damage." That combination — symptoms that can precede the blood abnormality, and damage that may not fully reverse — is why B12 symptoms warrant attention rather than watchful waiting.
What symptoms can and cannot do
Symptoms are useful for deciding whether to investigate. They are unreliable for deciding what you have. That distinction is the entire case for testing rather than guessing, and for bringing a specific description to a clinician rather than a self-diagnosis.
Our free assessment organizes your symptoms and risk factors into something specific enough to discuss with a doctor.
References
This article summarizes published guidance from the sources linked above. It is general information, not medical advice, and it cannot tell you whether you personally have a deficiency. Only a clinician ordering the right test can do that. If you have symptoms that concern you, speak to a doctor.