Free Testosterone: Which Tests to Order Alongside

What Free Testosterone Actually Measures

Free testosterone is the fraction of total testosterone not bound to sex hormone-binding globulin (SHBG) or albumin. It circulates unbound in serum, available to enter cells and activate androgen receptors directly. In adult males, free T accounts for only 2-3% of total testosterone, while about 44% binds tightly to SHBG and roughly 54% binds loosely to albumin [1].

This distinction matters because SHBG fluctuates independently of testosterone production. Obesity lowers SHBG, which can mask true deficiency by keeping total T in the normal range while bioavailable fractions drop [2]. Aging, liver disease, and hyperthyroidism raise SHBG, potentially producing low total T readings even when free T remains adequate [3]. The 2018 Endocrine Society Clinical Practice Guideline recommends measuring free or bioavailable testosterone whenever total testosterone is near the lower boundary of normal or when SHBG abnormalities are suspected [4]. Without free T, you risk misclassifying patients in either direction.

The gold-standard assay is equilibrium dialysis. Many commercial labs instead use a direct analog immunoassay, which the Endocrine Society has flagged as unreliable at low concentrations [4]. Calculated free testosterone, derived from total T, SHBG, and albumin using the Vermeulen equation, correlates well with dialysis results and is widely accepted in clinical practice [5].

Total Testosterone: The Obligatory Companion

You should never interpret free T in isolation. Total testosterone provides the denominator. A low free T with a normal total T points toward elevated SHBG as the mechanism. A low free T paired with a low total T confirms genuine hypogonadism and directs further investigation toward the hypothalamic-pituitary-gonadal axis [4].

The Endocrine Society defines male hypogonadism as a total testosterone consistently below 300 ng/dL on morning draws performed before 10:00 AM [4]. However, this threshold was derived from population studies using specific immunoassays, and inter-lab variability can be significant. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is the recommended method for total T measurement per the 2018 guideline [4]. Pair total T with free T and SHBG to triangulate: all three values together give you diagnostic confidence that a single number cannot.

The timing of the blood draw matters more than most patients realize. Testosterone follows a circadian rhythm, peaking between 6:00 and 9:00 AM in younger men, with a blunted but still present rhythm in men over 65 [6]. Draws taken in the afternoon can underestimate true levels by 20-25%.

SHBG and Albumin: The Binding Context

SHBG is not optional. It is the primary determinant of how much testosterone remains free. A 2010 study in the Journal of Clinical Endocrinology and Metabolism (N=3,220 community-dwelling men from the Framingham Heart Study) found that SHBG explained more variance in free testosterone than total testosterone itself in men over 50 [7].

Conditions that raise SHBG include aging, hepatic cirrhosis, hyperthyroidism, anticonvulsant use, and estrogen therapy. Conditions that lower SHBG include obesity, insulin resistance, type 2 diabetes, hypothyroidism, nephrotic syndrome, and androgen use [8]. If you skip SHBG, you cannot calculate free T (when equilibrium dialysis is unavailable), and you lose the single most informative variable for explaining discrepancies between total and free testosterone results.

Albumin completes the calculation. Most labs include albumin in a comprehensive metabolic panel, so ordering it separately is rarely necessary. The Vermeulen equation uses total T, SHBG, and albumin concentrations to produce a calculated free T that the Endocrine Society accepts as clinically equivalent to dialysis-measured free T in most settings [5].

LH and FSH: Primary vs. Secondary Hypogonadism

Once you confirm low testosterone, the next question is where the axis fails. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) answer that question directly [9].

High LH and FSH with low testosterone indicate primary hypogonadism: the testes are not responding to adequate gonadotropin signaling. Causes include Klinefelter syndrome, prior chemotherapy, orchitis, and age-related Leydig cell decline. Low or inappropriately normal LH and FSH with low testosterone indicate secondary (central) hypogonadism: the hypothalamus or pituitary is underproducing gonadotropin-releasing hormone or gonadotropins [4]. This pattern demands further workup, including pituitary MRI and prolactin measurement, because it can signal a pituitary adenoma, infiltrative disease, or hypothalamic suppression from opioids or obesity [10].

The American Association of Clinical Endocrinologists (AACE) 2020 algorithm for male hypogonadism places LH/FSH measurement immediately after confirmation of low testosterone on repeat testing [11]. Skipping these tests means starting TRT without knowing the etiology, which can delay diagnosis of a treatable pituitary mass.

Estradiol: The Overlooked Androgen Co-Marker

Estradiol (E2) should be part of every testosterone workup in men. Approximately 80% of circulating estradiol in males is produced by peripheral aromatization of testosterone in adipose tissue [12]. A sensitive estradiol assay (LC-MS/MS, not the standard immunoassay designed for female-range values) is preferred for male patients [13].

Why does estradiol matter here? Elevated E2 relative to testosterone can suppress gonadotropin secretion through negative feedback, contributing to secondary hypogonadism [12]. In men on TRT, rising estradiol is the mechanism behind gynecomastia, water retention, and mood changes. The TRT Trials (Testosterone Trials, N=790 men aged 65 and older) documented that testosterone gel treatment increased estradiol by a median of 10.8 pg/mL, with clinical effects that varied by baseline E2 [14].

Interpreting the testosterone-to-estradiol ratio helps clinicians decide whether aromatase inhibitor co-administration is warranted, though routine AI use remains controversial. The Endocrine Society does not recommend routine estradiol monitoring in all TRT patients but does recommend checking E2 when symptoms of estrogen excess appear [4].

Prolactin: When the Pituitary Needs a Closer Look

Prolactin should be ordered whenever LH and FSH return low or normal in the setting of low testosterone. Hyperprolactinemia suppresses GnRH pulsatility, causing secondary hypogonadism, and the most common pathologic cause is a prolactinoma [15]. A 2011 meta-analysis in Pituitary estimated that prolactinomas account for roughly 40% of all pituitary adenomas [16].

Mild prolactin elevations (25-50 ng/mL) can result from medications (antipsychotics, metoclopramide), stress from the blood draw itself (so-called "stalk effect" or venipuncture artifact), or macroprolactinemia. Values exceeding 200 ng/mL strongly suggest a macroprolactinoma and warrant gadolinium-enhanced pituitary MRI [15]. Ordering prolactin upfront prevents a second round of testing and a delayed referral to endocrinology.

CBC: Hematocrit and Polycythemia Surveillance

A complete blood count is required before starting TRT and at regular intervals afterward. Testosterone stimulates erythropoiesis via erythropoietin upregulation and direct effects on bone marrow stem cells [17]. The resulting rise in hematocrit is the most common adverse effect of TRT. In the Testosterone Trials, hematocrit exceeded 54% in 3.4% of men on testosterone gel versus 0.3% on placebo [14].

The Endocrine Society recommends checking hematocrit at baseline, at 3-6 months after initiation, and annually thereafter [4]. A hematocrit above 54% should prompt dose reduction, temporary discontinuation, or therapeutic phlebotomy [4]. Baseline CBC also screens for anemia, which can be a presenting sign of hypogonadism itself. A 2006 study in the Archives of Internal Medicine found that 30% of men with unexplained normocytic anemia had undiagnosed testosterone deficiency [18].

Metabolic Panel and Lipids: Cardiovascular and Hepatic Baseline

A comprehensive metabolic panel (CMP) captures fasting glucose, electrolytes, renal function, and hepatic transaminases. Testosterone deficiency is independently associated with insulin resistance, metabolic syndrome, and type 2 diabetes [19]. The 2020 AACE/ACE consensus statement on male hypogonadism recommends baseline glucose and HbA1c as part of the diagnostic workup [11].

Fasting lipids should also be drawn. The relationship between testosterone and lipid profiles is dose-dependent and nonlinear. The TRAVERSE trial (N=5,246, mean age 63) confirmed that testosterone replacement did not increase major adverse cardiovascular events over a median 33-month follow-up, but it did produce small decreases in HDL cholesterol [20]. Baseline lipids let clinicians monitor this shift and contextualize cardiovascular risk before and during therapy.

Liver function tests matter because oral testosterone formulations (now available as testosterone undecanoate capsules) undergo first-pass hepatic metabolism, and because conditions elevating SHBG often include hepatic pathology [8]. If transaminases are elevated at baseline, you need to know that before attributing later rises to therapy.

Thyroid Function and DHEA-S: Conditional Add-Ons

Thyroid-stimulating hormone (TSH) and free T4 are not part of every testosterone panel, but they should be ordered when SHBG is unexpectedly high or low. Hyperthyroidism raises SHBG and can mimic hypogonadism on total T testing, while hypothyroidism lowers SHBG and may mask deficiency [21]. A quick TSH screen avoids this diagnostic trap.

DHEA-S measures adrenal androgen production. It becomes relevant in women with hyperandrogenism (where the source of excess testosterone may be adrenal rather than ovarian) and in men with suspected adrenal insufficiency [22]. For a standard male hypogonadism workup, DHEA-S is not routinely necessary. Add it when cortisol or adrenal pathology is in the differential.

How to Raise Free Testosterone

For men with confirmed low free testosterone, the approach depends on etiology. Weight loss is the single most effective non-pharmacologic intervention. A 2019 meta-analysis in the European Journal of Endocrinology (11 RCTs, N=1,031) found that diet-induced weight loss of 9.8% body weight increased total testosterone by a mean of 2.9 nmol/L in men with obesity [23].

Resistance training raises free testosterone acutely, with post-exercise elevations lasting 15-30 minutes. Chronic resistance training over 12 weeks has been shown to increase resting free testosterone modestly in previously sedentary men [24]. Sleep optimization matters too: a University of Chicago study restricted healthy young men to 5 hours of sleep for one week and documented a 10-15% decline in daytime testosterone [25].

Pharmacologically, TRT (injectable testosterone cypionate 100-200 mg weekly, or topical gels at 50-100 mg daily) reliably raises free T into the normal range. Clomiphene citrate 25-50 mg daily is an off-label alternative that preserves fertility by stimulating LH release [26]. Enclomiphene, its trans-isomer, is under investigation for the same purpose.

How to Lower Free Testosterone

Elevated free testosterone in women is the hallmark of polycystic ovary syndrome (PCOS). The 2023 international evidence-based PCOS guideline recommends calculated free T or equilibrium dialysis free T as the most sensitive biochemical marker for hyperandrogenism [27].

First-line pharmacologic treatment to lower free T in PCOS is a combined oral contraceptive pill, which raises SHBG and directly suppresses ovarian androgen production [27]. Spironolactone 50-200 mg daily acts as an androgen receptor antagonist and is commonly added when hirsutism persists [28]. Metformin lowers free testosterone indirectly by improving insulin sensitivity, which reduces insulin-driven ovarian androgen synthesis [29].

In men, elevated free testosterone is rare outside exogenous androgen use. If a male patient shows a high free T with suppressed LH and FSH, undisclosed anabolic steroid use should be considered and addressed directly.

Putting the Panel Together: A Practical Ordering Guide

For a male patient presenting with symptoms consistent with hypogonadism (low libido, erectile dysfunction, fatigue, reduced muscle mass), order the following as a first-round morning draw:

Total testosterone (LC-MS/MS preferred), free testosterone (equilibrium dialysis or calculated), SHBG, albumin (usually included in CMP), LH, FSH, estradiol (sensitive assay), prolactin, CBC with differential, comprehensive metabolic panel, fasting lipid panel, and HbA1c. Add TSH/free T4 if SHBG is outside the expected range or thyroid symptoms are present.

If the first total T is below 300 ng/dL, repeat the morning draw on a separate day before making a diagnosis [4]. This two-draw requirement catches transient dips from acute illness, poor sleep, or stress. Once confirmed, the LH/FSH result tells you whether to image the pituitary or evaluate the testes directly.

For TRT monitoring, the Endocrine Society recommends total T and free T at 3 months (trough level for injectables), hematocrit at 3-6 months, PSA at 3-12 months in men over 40, and a bone density scan at 1-2 years if osteoporosis was present at baseline [4].