What’s the difference between total and free testosterone?

Testosterone is a vital hormone for both men and women. But if you’ve ever taken a testosterone blood test before, you’ve probably noticed there are two different types: total and free. Both can be helpful to establish whether your testosterone levels are low, but what’s the difference? 

If you want a quick answer about which test is right for you, jump to should I test free or total testosterone?

In this article, we cover:  

• What is testosterone?  
• Testosterone in the bloodstream 
• Free vs total testosterone 
• How is free testosterone calculated?
• What is bioavailable testosterone? 
• What is a normal free testosterone level? 
• Can you have normal total testosterone but low free testosterone? 
• Can you have low total testosterone but normal free testosterone?
• How can I increase free testosterone levels?  
• Should I test free or total testosterone? 
   

What is testosterone?

First, let’s recap testosterone’s overall function in the body.  

Testosterone is a primary sex hormone primarily made in the testes, but also in the ovaries and adrenal glands. It’s important for growth and development, fertility, and maintaining muscle and bone mass. Low levels can affect mood, energy levels, and sexual function.   

The effects of testosterone are more clearly seen in men, but the hormone has important roles in women too. Abnormal levels, low or high, can give rise to symptoms and may be due to an underlying condition.  

Learn more about the role of testosterone in women.  
 

Testosterone in the bloodstream

Once testosterone is made, it’s released into the bloodstream. But because it doesn’t dissolve in water (or blood), most testosterone (about 95%) catches a ride on other proteins, like sex hormone-binding globulin (SHBG) and albumin.

All the time testosterone is bound to proteins, it’s rendered inactive. It can’t act on tissues until it’s released. There are two benefits to this: 

• It regulates testosterone activity 
• It prevents the liver and kidneys from deactivating and removing testosterone too soon 

Once it reaches its target cell, testosterone binds to specific receptors and triggers changes within the cell.  
 

Free vs total testosterone

As we’ve seen, carrier proteins bind to and release testosterone within the bloodstream.  

Total testosterone is a total measure of testosterone in the bloodstream — both free and bound. Levels are measured by an analyser directly.  

On the other hand, free testosterone is the concentration of testosterone in the blood that is not attached to proteins. Only about 2–5% of testosterone is free.  
 

How is free testosterone calculated?

Measuring free testosterone directly is possible, but it’s challenging and not widely available. So it’s often calculated instead, using total testosterone, SHBG, and albumin levels. It’s a complicated equation that produces a very reliable estimate. Thankfully, the lab works this out for you.  
 

Free testosterone in women 
 

In women, levels of testosterone are around ten times lower than men, and the majority is bound to the protein, SHBG (about 80%). As such, free androgen index (FAI) is a better measure than free testosterone. It’s the ratio of total testosterone to SHBG and is useful for detecting raised androgens (male hormones), as is seen in conditions like polycycstic ovary syndrome (PCOS). 
 

Total testosterone refers to all forms of testosterone in the bloodstream. Free testosterone is any testosterone that is not bound to a protein. Free androgen index is a measure of free testosterone in women.   

 

What is bioavailable testosterone?

Bioavailable testosterone reflects testosterone that is readily available and able to act to tissues. It makes sense that free testosterone falls into this category, since it can bind to cell receptors as it pleases. 

But testosterone bound to albumin is also classed as bioavailable. That’s because the testosterone-albumin bond is easily reversible, meaning testosterone frequently switches between active and inactive forms. This effect is greatest in men as a greater proportion of their testosterone binds to albumin.    

The remaining testosterone is bound to SHBG and this bond is more secure. This means that high levels of SHBG can reduce levels of free and bioavailable testosterone.  

Bioavailable testosterone is a measure of testosterone that is readily available to act on tissues. It is calculated by adding free testosterone to albumin-bound testosterone.

 

What is a normal free testosterone level?

Normal free testosterone levels for men and women vary according to the laboratory. The table below is given as a guide.  

	Men	Women
Total testosterone (nmol/L)	8.7–29	Age 20 – 49: 0.29–1.67 Age 50 or over: 0.1–1.42
Free testosterone (nmol/L)	0.2–0.62	0.001–0.02
Free androgen index (%)	Rarely measured	0.18–7.07
Albumin (g/L)	35–50	35–50
SHBG (nmol/L)	18.3–54	27–146

 Testosterone levels vary according to life stage. Find out more about how testosterone levels decline with age. 
 

Can you have normal total testosterone and low free testosterone?

Yes, it’s possible to have normal total testosterone levels but low free testosterone, which may lead to symptoms of testosterone deficiency.  

As we’ve seen, it’s the bioavailable portion of testosterone that exerts its effects on the body. So, if SHBG levels are particularly raised relative to total testosterone, it won’t leave much free testosterone to act on tissues. This can be seen in older age, where SHBG levels often increase considerably [1].  

Other causes of raised SHBG levels include [2]:  

• Medications — such as oestrogens, anti-epilepsy drugs, and metformin 
• Conditions — such as liver disease, an overactive thyroid (hyperthyroidism), and HIV 
   

Can you have low total testosterone but normal free testosterone levels?

The opposite is also true: total testosterone levels may appear low when free testosterone levels are within normal range. This is most often seen in men in the development of insulin resistance (obesity or diabetes), which often causes SHBG levels to decline. Therefore, less testosterone is bound to proteins [1]. You can check your blood sugar control from home with our Diabetes (HbA1c) Blood Test.
 

How can I increase free testosterone levels?

Free testosterone levels are dependent on both total testosterone levels and your levels of SHBG (the protein that binds to it).  

Therefore, increasing your total testosterone level through healthy lifestyle choices is also likely to increase your free testosterone levels. Factors like stress, low levels of activity, and being overweight can all impact your testosterone production. There are many ways you can boost testosterone levels naturally, from losing weight to maintaining a healthy, balanced diet.  

SHBG naturally increases with age, but you may be able to reduce this effect somewhat by making sure your diet contains plenty of protein [9]. If your levels are abnormally high, it’s worth exploring whether medications or an underlying health condition may be playing a part in this. 
 

Should I test total or free testosterone?

The answer to this question will depend on why you’re testing and your gender.

Men 

If you’re curious, or just want to make sure you’re not deficient, total testosterone will likely give you your answer. This is usually the first step in investigating symptoms of hypogonadism (low testosterone) and makes a great screening test.  

If you have low or borderline total testosterone levels or you have ongoing symptoms, your doctor may advise further testing, which may include free testosterone, SHBG, LH (to distinguish between different types of hypogonadism), or FH (if fertility is an issue) [3]. You can check your free testosterone levels at home with our finger-prick Free Testosterone Blood Test, or if you'd also like to test for the other hormones mentioned, try our Male Hormone Blood Test. 

There is some evidence that free testosterone may be more useful when investigating hypogonadism [1] for reasons mentioned above — total testosterone may be normal even when free testosterone is low.  

Women 

When investigating causes of hyperandrogenism in women, such as PCOS, total testosterone, SHBG, and free androgen index are most useful. Other hormones like luteinising hormone (LH), follicle-stimulating hormone (FSH) and prolactin are sometimes also measured to rule out other causes of irregular periods.  

Some women suspect their testosterone levels are low because they have low libido, but the two do not always correlate. It’s likely that the metabolism of testosterone, rather than absolute levels, is a more important determinant of libido, which can’t be measured [4].  

Where testosterone replacement is being considered, a total testosterone level and FAI measurement are useful to make sure testosterone levels are not already at the upper normal limit.   

Trans and non-binary people on hormone therapy 

If you’re looking to monitor your hormone regime, the Endocrine Society recommends testing total testosterone levels, alongside other hormones such as oestradiol and prolactin in trans women. Guidelines suggest monitoring hormone therapy every three months during the first year (that is, with each dose adjustment), then once or twice a year once the dose is stabilised [5–7]. That said, free or bioavailable testosterone measurements may be valuable if total testosterone levels don’t seem to align with the stage of transition [8], which may be due to raised SHBG levels.  

When taking hormone medication, it’s important to make sure you’re taking your blood test at the right time. Gender GP provides a helpful guide of blood test timings. And if you use hormone gels and are taking a finger-prick test, you should make sure your finger is completely clear of any hormone gel to avoid an abnormally high result, or ideally take a venous test. 
 

The bottom line 

Testosterone is more complicated that it seems at first glance. To summarise, total testosterone is a helpful biomarker to measure if you’re experiencing symptoms of low or raised levels. This can be combined with free testosterone or a free androgen index for a better idea of bioavailable testosterone levels.   

 

---

References 

1. Antonio L, Wu FCW, O’Neill TW, Pye SR, Ahern TB, Laurent MR, et al. Low Free Testosterone Is Associated with Hypogonadal Signs and Symptoms in Men with Normal Total Testosterone. The Journal of Clinical Endocrinology & Metabolism. 2016 Jul 1;101(7):2647–57.
2. Bhasin S, Brito JP, Cunningham GR, Hayes FJ, Hodis HN, Matsumoto AM, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018 May 1;103(5):1715–44.
3. Hackett G, Kirby M, Edwards D, Jones TH, Wylie K, Ossei-Gerning N, et al. British Society for Sexual Medicine Guidelines on Adult Testosterone Deficiency, With Statements for UK Practice. J Sex Med. 2017 Dec;14(12):1504–23.
4. Mariette-JB. Testosterone replacement in menopause [Internet]. British Menopause Society. 2019 [cited 2023 Feb 1]. Available from: https://thebms.org.uk/2019/02/testosterone-replacement-in-menopause/
5. Unger CA. Hormone therapy for transgender patients. Transl Androl Urol. 2016 Dec;5(6):877–84.
6. Hembree WC, Cohen-Kettenis PT, Gooren L, Hannema SE, Meyer WJ, Murad MH, et al. Endocrine Treatment of Gender-Dysphoric/Gender-Incongruent Persons: An Endocrine Society* Clinical Practice Guideline. The Journal of Clinical Endocrinology & Metabolism. 2017 Nov 1;102(11):3869–903.
7. Blood Test Orders [Internet]. GenderGP Transgender Services. [cited 2023 Feb 1]. Available from: https://www.gendergp.com/help-centre/blood-test-orders/
8. Maheshwari A, Nippoldt T, Davidge-Pitts C. An Approach to Nonsuppressed Testosterone in Transgender Women Receiving Gender-Affirming Feminizing Hormonal Therapy. J Endocr Soc. 2021 Apr 16;5(9):bvab068.
9. Longcope C, Feldman HA, McKinlay JB, Araujo AB. Diet and sex hormone-binding globulin. J Clin Endocrinol Metab. 2000 Jan;85(1):293–6.
10. Testosterone test kit - Medichecks