What does a female patient’s testosterone level of 0.6 nmol/L, sex‑hormone‑binding globulin of 140 nmol/L (elevated), and free androgen index of 0.4 % indicate?

Laboratory Interpretation: Low Bioavailable Androgens Despite Normal Total Testosterone

These results indicate reduced androgen bioavailability due to markedly elevated SHBG (140 nmol/L), which binds most of the circulating testosterone and results in a low Free Androgen Index (0.4%), despite normal total testosterone levels. This pattern suggests that the biologically active testosterone available to tissues is at the lower end of normal or potentially insufficient.

Understanding the Results

The Core Problem: SHBG Elevation

Your SHBG is significantly elevated at 140 nmol/L (reference 30-110 nmol/L). SHBG acts as a high-affinity binding protein that tightly binds testosterone, rendering the SHBG-bound fraction biologically unavailable to tissues 1. When SHBG is elevated, even normal total testosterone levels can result in reduced free (bioactive) testosterone.

What Each Value Means

• Total Testosterone (0.6 nmol/L): This is in the lower-normal range but technically within reference limits (0.2-1.8 nmol/L)

• SHBG (140 nmol/L): Markedly elevated, approximately 27% above the upper limit of normal. SHBG levels are raised by estrogen, thyroid hormones, certain medications (anticonvulsants like valproic acid), liver disease, and hyperthyroidism 1, 2

• Free Androgen Index (0.4%): This calculated ratio (total testosterone/SHBG × 100) reflects bioavailable androgen activity. Your value of 0.4% is at the lower end of the reference range (0.3-4.0%), indicating minimal free testosterone available for tissue action 3

Clinical Significance

Androgen Bioavailability

The Free Androgen Index and calculated free testosterone are more sensitive indicators of androgen status than total testosterone alone, particularly when SHBG is abnormal 1, 4. Your low FAI suggests that despite "normal" total testosterone, the amount of biologically active testosterone reaching target tissues (hair follicles, skin, bone, muscle, brain) is reduced.

Research demonstrates that FAI values below 1.0 indicate mild androgenization without elevation of free testosterone, whereas values above 1.0 suggest absolute hyperandrogenism 5. Your FAI of 0.4% falls well below this threshold, consistent with low androgen bioactivity.

What Could Cause Elevated SHBG?

Common causes to investigate include:

• Hyperthyroidism (most important to exclude)
• Estrogen-containing medications or hormone therapy
• Certain anticonvulsants (particularly valproic acid/Depakine®) 2
• Medications with estrogenic effects (tamoxifen, raloxifene)
• Liver disease (particularly primary biliary cirrhosis or portal hypertension) 2
• Genetic factors (rare SHBG gene mutations)
• Low body weight or eating disorders

Recommended Next Steps

Essential Investigations

1. Thyroid function testing (TSH, free T4) - this is the most critical test given the marked SHBG elevation 1

2. Medication review - identify any drugs that increase SHBG 2

3. Liver function tests - to exclude hepatic causes of elevated SHBG 2

4. Consider calculated free testosterone using a validated calculator (Vermeulen method) rather than direct immunoassay, which is notoriously inaccurate 1, 6

Clinical Context Matters

If you have symptoms potentially related to low androgen bioavailability (fatigue, reduced libido, difficulty building muscle, mood changes, bone health concerns), these laboratory findings provide biochemical support for those symptoms 7. The elevated SHBG is independently associated with androgen deficiency features, even when total testosterone appears normal 7.

If you have no symptoms, this may simply represent your individual physiology, though the underlying cause of elevated SHBG should still be identified.

Important Caveats

Assay Limitations

Automated testosterone immunoassays available in most hospital laboratories are often inadequate for accurately measuring testosterone in women 1. The low concentrations in women challenge assay sensitivity and specificity. If clinical suspicion for androgen abnormality persists, consider repeat testing using liquid chromatography-tandem mass spectrometry (LC-MS/MS) in a specialty laboratory 1, 4.

Reference Range Issues

Reference ranges for female androgens are problematic because they are often derived from populations that may include women with unrecognized androgen excess 1. Additionally, there is no uniform laboratory standard for free testosterone or FAI, so results differ widely between laboratories 1.

Timing Considerations

For premenopausal women with regular cycles, androgen testing should ideally be performed in the early morning during the follicular phase (days 4-10) when testosterone levels are most stable and standardized 1. Testosterone levels vary by approximately 25% around the mean during the menstrual cycle 1.

Bottom Line

Your results show normal total testosterone but reduced bioavailable androgens due to markedly elevated SHBG. The priority is identifying why your SHBG is elevated—particularly excluding hyperthyroidism, which is the most common and clinically significant cause 1. The clinical significance depends on whether you have symptoms consistent with reduced androgen bioactivity and what underlying condition is driving the SHBG elevation.