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Free Testosterone Calculator

Free Testosterone Calculator

Understanding Free Testosterone

Free testosterone is the biologically active form of testosterone that is not bound to proteins in the blood. While total testosterone measurements are more common, free testosterone levels can provide valuable information about the hormone's availability to tissues. This calculator uses the Vermeulen method to estimate free and bioavailable testosterone levels.

What is Free Testosterone?

Free testosterone represents only about 2-3% of total testosterone in the blood. The majority of testosterone is bound to either:

  • Sex Hormone Binding Globulin (SHBG) - about 60%
  • Albumin - about 38%

Clinical Significance

Free testosterone measurement is important for:

  • Evaluating androgen disorders
  • Diagnosing hypogonadism
  • Monitoring testosterone replacement therapy
  • Assessing fertility issues
  • Evaluating certain endocrine conditions

Normal Ranges

Free testosterone levels vary by:

  • Age
  • Sex
  • Time of day
  • Laboratory method

Typical ranges for adult males:

  • 1-3% of total testosterone
  • 0.17-0.77 nmol/L
  • 5-21 ng/dL

When to Test Free Testosterone

Consider testing when:

  • Total testosterone results are borderline
  • SHBG levels are abnormal
  • Symptoms of low testosterone despite normal total testosterone
  • Monitoring hormone replacement therapy
  • Evaluating infertility

Factors Affecting Results

Several factors can influence free testosterone levels:

  • Age
  • Body mass index (BMI)
  • Medical conditions
  • Medications
  • Time of day
  • Stress
  • Exercise
  • Diet

Important Considerations

This calculator:

  • Uses the validated Vermeulen equation
  • Assumes normal albumin (43 g/L)
  • Requires accurate total testosterone and SHBG values
  • Should not be used in pregnancy
  • May need albumin adjustment in certain conditions

When to Use Caution

Special considerations are needed for:

  1. Pregnancy

    • Calculator will give falsely low results
    • High estradiol affects SHBG binding
  2. Abnormal Albumin Levels

    • Measure actual albumin if levels may be abnormal
    • Common in:
      • Pregnancy
      • Liver disease
      • Nephrotic syndrome
  3. Laboratory Values

    • Use morning testosterone values
    • Ensure accurate SHBG measurement
    • Consider repeat testing if results are unexpected

Clinical Applications

Free testosterone calculation is useful for:

  • Diagnosis of hypogonadism
  • Monitoring treatment response
  • Evaluating androgen disorders
  • Assessing male infertility
  • Research purposes

Start calculating now to better understand your testosterone status!

References:

  1. Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999;84(10):3666-3672.