What Is a Free Testosterone Blood Test?
Free testosterone is the unbound, biologically active form — only 1–3% of total testosterone.
Why Test Free T?
- Total T can be misleading
- SHBG binds testosterone — high SHBG = low free T
- Symptoms may reflect free T, not total T
Factors Affecting SHBG
- High SHBG: aging, liver disease, hyperthyroidism, estrogen
- Low SHBG: obesity, insulin resistance, hypothyroidism
When to Test Free T
- Symptoms of low T but total T is borderline
- Conditions affecting SHBG
- Monitoring TRT effectiveness
Source: Endocrine Society, NLM MedlinePlus.
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