Evidence check: FT4 in the context of fatigue

FT4 (free thyroxine) is a key thyroid marker. Low FT4 levels can indicate hypothyroidism, a condition strongly associated with fatigue, lethargy, and low energy. The link between hypothyroidism and fatigue is well-established in clinical medicine (evidence: human-strong). However, FT4 alone is insufficient for diagnosis; TSH and often T3 should also be measured. Fatigue has many other potential causes (iron deficiency, sleep disorders, stress, inflammation). A MyBody-X test does not measure FT4 directly; it analyzes genetic variants that may influence thyroid metabolism, but the clinical impact is usually small. For persistent fatigue with suspected thyroid issues, a medical blood test is the first step, not a genetic test. Caveat: Genetic tests cannot replace standard lab diagnostics; evidence for specific SNPs affecting FT4 and fatigue is mechanistic or weak.