Low Libido — Blood Tests for Men and Women
A drop in libido is rarely just “in your head.” Hormonal, nutritional, and metabolic causes show up on routine blood tests — and most are treatable.
The Short Version
Libido depends on a complex interplay of hormones (testosterone, oestrogen, prolactin, thyroid), neurotransmitters (dopamine, serotonin), nutrients (iron, vitamin D, B12), general health, mood, and relationship factors. When desire drops persistently, blood tests can identify treatable biological contributors in a substantial proportion of cases.
For men, the standard panel is morning testosterone (8–10 am), SHBG, LH, FSH, prolactin, TSH, ferritin, and vitamin D. For women, it is oestradiol, FSH, LH, testosterone, SHBG, prolactin, TSH, ferritin, and vitamin D. All are bulk-billed under Medicare with appropriate clinical indication.
Common findings: low testosterone in men over 40, perimenopause in women in their 40s, iron deficiency in menstruating women, hypothyroidism, and medication side effects (especially SSRIs and the combined oral contraceptive pill). All of these are addressable.
10 Causes of Low Libido Your Blood Can Reveal
Low Testosterone (Men)
How it lowers libido: Testosterone is the primary driver of male sexual desire — it acts on dopamine pathways in the brain, supports the physical capacity for erections, and maintains energy and mood. Levels naturally decline by about 1% per year after age 30, but pathological hypogonadism (testosterone below 8 nmol/L) is far more common than most men realise — affecting roughly 1 in 5 men over 60. Testosterone is highest in the morning, so a level taken at 4 pm can easily be misleadingly low or normal.
Typical clinical pattern:
Reduced sexual interest plus reduced morning erections, harder time achieving or maintaining erections, fatigue (especially afternoon energy crashes), low mood or irritability, loss of muscle mass, increased belly fat, and in some men, gynaecomastia (breast tissue growth). Often comes on gradually — partner may notice the change first.
Next step: Ask for a fasting morning testosterone (between 8 and 10 am) on two separate days. Australian normal is 10–30 nmol/L. Below 8 with symptoms is hypogonadism. SHBG and LH/FSH help work out whether the testes or the pituitary is the source.
Low Oestrogen / Perimenopause and Menopause (Women)
How it lowers libido: Oestrogen and testosterone both contribute to female sexual desire and physical arousal. As ovarian function declines through perimenopause (typically starting in the early 40s), oestradiol fluctuates and then drops, FSH rises as the pituitary tries to compensate, and ovarian testosterone production decreases. Falling oestrogen also causes vaginal dryness and thinning of vaginal tissue, making sex physically uncomfortable.
Typical clinical pattern:
Reduced sexual interest plus hot flushes, night sweats, irregular or skipped periods, vaginal dryness or discomfort during sex, sleep disturbance, mood changes, brain fog, and joint aches. Average age of menopause in Australia is 51, but perimenopause can start a decade earlier.
Next step: In women still having periods, blood tests on day 2–5 of the cycle help (FSH > 25 IU/L on two occasions suggests perimenopause). In post-menopausal women, low oestradiol (typically below 70 pmol/L) and elevated FSH (above 30 IU/L) confirm menopause. Treatment options include menopausal hormone therapy (MHT/HRT), vaginal oestrogen, and increasingly low-dose testosterone for women with persistent low libido.
Elevated Prolactin (Hyperprolactinaemia)
How it lowers libido: Prolactin is made by the pituitary gland, normally to support breastfeeding. Elevated prolactin outside pregnancy directly suppresses the release of GnRH (the master hormone for sex hormone production), which in turn lowers testosterone in men and oestrogen in women. Causes include a benign pituitary tumour (prolactinoma), certain medications (antipsychotics, metoclopramide, some antidepressants), an underactive thyroid, severe stress, and chronic kidney disease.
Typical clinical pattern:
Low libido in both sexes. In women: irregular or absent periods, milky nipple discharge (galactorrhoea), infertility. In men: erectile dysfunction, reduced facial hair, occasionally galactorrhoea. Headaches and visual disturbance suggest a pituitary tumour pressing on the optic nerve — get this checked urgently.
Next step: A single prolactin level is enough to screen. Normal is below 500 mIU/L (women) or 350 mIU/L (men) in most Australian labs. Mildly elevated levels warrant a repeat with macroprolactin testing. Sustained high prolactin needs referral for pituitary MRI.
Hypothyroidism (Underactive Thyroid)
How it lowers libido: Thyroid hormones regulate the metabolic rate of every tissue, including the brain regions involved in sexual desire. Hypothyroidism reduces dopamine and serotonin signalling, raises SHBG (which binds testosterone and reduces free hormone), can elevate prolactin, and causes profound fatigue that further dampens libido.
Typical clinical pattern:
Reduced libido plus fatigue, weight gain (often despite no change in eating), feeling cold, dry skin, hair thinning, constipation, slow thinking, low mood, heavier or irregular periods in women, and muscle aches. Hashimoto’s thyroiditis is the most common cause in Australia and runs in families.
Next step: TSH alone is the screening test. Normal is roughly 0.4–4.0 mIU/L. TSH above 4.0 with low Free T4 confirms overt hypothyroidism. Treatment with levothyroxine usually restores libido within 6–12 weeks of reaching target TSH.
Iron Deficiency
How it lowers libido: Iron deficiency directly reduces dopamine receptor function in the brain — the same neurotransmitter system that drives sexual desire and reward. Beyond the obvious fatigue, iron-deficient people often describe a generalised loss of motivation and pleasure-seeking. Importantly, this happens long before haemoglobin drops into the anaemic range. Ferritin below 30 mcg/L is enough to cause symptoms.
Typical clinical pattern:
Low libido plus fatigue (worse in the afternoon), reduced exercise tolerance, hair thinning, brittle nails, restless legs at night, brain fog. Very common in women of reproductive age (heavy periods), pregnancy, plant-based eaters, and people with coeliac disease or inflammatory bowel disease.
Next step: Ferritin is the single most useful test. Australian labs typically report 30–300 mcg/L as normal but most clinicians treat ferritin below 50 in symptomatic women and below 100 in athletes. Iron studies and a full blood count round out the panel. All bulk-billed.
Vitamin D Deficiency
How it lowers libido: Vitamin D receptors are found in the testes, ovaries, and brain regions involved in mood and motivation. Low vitamin D is associated with lower testosterone in men and possibly reduced sexual function in women. Vitamin D also affects mood — low levels are linked to depression, which independently lowers libido. Despite the Australian sunshine, deficiency is common.
Typical clinical pattern:
Reduced libido often accompanied by fatigue, low mood, muscle aches, bone tenderness, and frequent minor infections. Symptoms are non-specific, so the diagnosis is essentially a blood test plus risk factors.
Next step: Ask for 25-hydroxyvitamin D. Australian labs report below 50 nmol/L as deficient, 50–75 as insufficient, 75–150 as adequate. Treatment with 1000–2000 IU daily of vitamin D3 typically restores levels in 8–12 weeks.
Antidepressants and Other Medications
How it lowers libido: SSRIs and SNRIs cause sexual side effects in 30–70% of users — reduced libido, delayed orgasm, erectile dysfunction. The mechanism is serotonin-mediated suppression of dopamine pathways. Other culprits include beta-blockers, spironolactone, some antipsychotics, opioids, finasteride for hair loss or BPH, and oral contraceptives in some women (raise SHBG, lower free testosterone).
Typical clinical pattern:
Libido drops within weeks to months of starting (or increasing the dose of) the offending medication. May persist long-term. A medication review with your GP is the diagnostic step.
Next step: Bring your full medication list (including supplements and over-the-counter) to the GP. Often a switch to a different antidepressant (bupropion, mirtazapine, vortioxetine) or a dose reduction resolves the issue. Never stop antidepressants suddenly without medical guidance.
Elevated SHBG (Bound Testosterone)
How it lowers libido: Sex hormone binding globulin (SHBG) is a protein made by the liver that binds testosterone tightly, making it inactive. When SHBG is high, total testosterone may look normal, but FREE (active) testosterone can be very low — explaining low libido despite a “normal” testosterone result. Causes of high SHBG include hyperthyroidism, liver disease, ageing, low body weight, oestrogen exposure (oral contraceptive pill in women), anti-epileptic drugs, and HIV.
Typical clinical pattern:
Symptoms of low testosterone (low libido, fatigue, low mood, reduced muscle mass) despite a total testosterone in the normal range. The diagnosis is missed if SHBG is not measured. Particularly common in lean older men and in women on the combined oral contraceptive pill.
Next step: If your total testosterone is in the lower half of normal but you have symptoms, ask specifically for SHBG. Free testosterone can then be calculated. Treatment depends on the cause — adjusting medications, treating thyroid or liver issues, or in some cases supplementing testosterone.
Chronic Illness (Liver, Kidney, Diabetes)
How it lowers libido: Chronic kidney disease, advanced liver disease, and poorly controlled diabetes all suppress the hypothalamic-pituitary-gonadal axis, lower testosterone, raise prolactin, and cause widespread fatigue and inflammation. Diabetes additionally damages blood vessels and nerves needed for arousal and erection. Chronic inflammation (raised CRP) of any cause reduces libido through cytokine effects on the brain.
Typical clinical pattern:
Low libido as part of generalised symptoms — fatigue, weight changes, swollen ankles, foamy urine (kidneys), itching or jaundice (liver), increased thirst and urination (diabetes). The libido issue often resolves significantly when the underlying condition is better controlled.
Next step: A general health screen — UEC, eGFR, LFTs, HbA1c, CRP, FBC — alongside the hormone panel identifies underlying contributors. All bulk-billed.
Depression and Inflammation
How it lowers libido: Depression directly reduces sexual desire through lowered dopamine and serotonin signalling, fatigue, and anhedonia (loss of pleasure). There is no “depression blood test,” but biological contributors are very common: low vitamin D, low B12, iron deficiency, hypothyroidism, and chronic low-grade inflammation (CRP > 3 mg/L). Treating these often improves both mood and libido.
Typical clinical pattern:
Low libido plus persistent low mood, loss of interest in usual activities, fatigue, sleep changes (early waking or oversleeping), changes in appetite, feelings of worthlessness, and difficulty concentrating.
Next step: A full screen — TSH, vitamin D, B12, ferritin, CRP, FBC — plus a conversation with your GP about mood. If depression is diagnosed, choosing an antidepressant with lower sexual side effects (bupropion, mirtazapine, vortioxetine, agomelatine) is reasonable.
Tests to Ask Your GP For
All routinely available through Sonic, Healius, Australian Clinical Labs, and other Australian pathology providers. Bulk-billed under Medicare with appropriate clinical indication. Tests marked with * have specific Medicare criteria (most libido and fertility workups qualify).
| Test | Why It Matters | Cost (Australia) |
|---|---|---|
| Total Testosterone (morning) | Primary driver of libido in men, contributes in women | Bulk billed |
| Free Testosterone or SHBG | Calculates the active fraction | Bulk billed* |
| LH and FSH | Locates the source of low sex hormones | Bulk billed |
| Oestradiol | Key in women - perimenopause and menopause | Bulk billed |
| Prolactin | High levels suppress sex hormones in both sexes | Bulk billed |
| TSH | Hypothyroidism is a common reversible cause | Bulk billed |
| Ferritin + Iron Studies | Iron deficiency affects dopamine and energy | Bulk billed |
| Vitamin D (25-OH) | Low D linked to lower testosterone, low mood | Bulk billed* |
| Vitamin B12 | Energy and neurotransmitter synthesis | Bulk billed |
| CRP | Inflammation contributes to low desire | Bulk billed |
| HbA1c | Diabetes affects nerves, vessels, and hormones | Bulk billed |
| LFTs and eGFR | Liver and kidney disease lower libido | Bulk billed |
| AMH (women) | Ovarian reserve - useful around perimenopause | Out of pocket |
| DHEAS | Adrenal androgen, contributes to female libido | Bulk billed* |
Low Libido + Other Symptoms Matcher
| Low Libido Plus... | Likely Cause | Test First |
|---|---|---|
| Reduced morning erections, fatigue (man) | Low testosterone | AM testosterone, SHBG, LH |
| Hot flushes, irregular periods | Perimenopause / menopause | FSH, oestradiol |
| Milky discharge, irregular periods, headache | High prolactin | Prolactin |
| Cold intolerance, weight gain, fatigue | Hypothyroidism | TSH, Free T4 |
| Heavy periods, hair thinning, fatigue | Iron deficiency | Ferritin, FBC |
| Started on SSRI / SNRI | Medication side effect | Med review + prolactin |
| On combined oral contraceptive pill | High SHBG / low free testosterone | SHBG, total testosterone |
| Persistent low mood, anhedonia | Depression + inflammation | TSH, Vit D, B12, CRP |
| Increased thirst, urination, weight loss | Diabetes | HbA1c, glucose |
| Vaginal dryness, painful sex | Low oestrogen / GSM | Oestradiol, FSH |
When Low Libido Needs Urgent Attention
Low libido itself is rarely an emergency, but certain accompanying symptoms can point to conditions that need prompt assessment.
What to Say to Your GP
GPs hear about libido concerns daily — you do not need to be embarrassed. Being specific saves time and gets the right tests.
“For the past [duration], my sex drive has dropped significantly compared with my baseline. I've also noticed [list other symptoms: e.g., fatigue, mood changes, period changes, weight changes]. I'd like to check whether there's a hormonal or nutritional cause. Could we do a hormone panel including [testosterone / oestradiol / FSH], TSH, prolactin, ferritin, and vitamin D?”
Helpful information to bring:
- Your full medication list (including the contraceptive pill, antidepressants, blood pressure meds, hair loss meds)
- Period history (women) — cycle length, flow, last period
- Other symptoms — energy, mood, sleep, weight, hair, skin
- When the change started and any obvious triggers (medication, stress, illness)
- Family history of thyroid, diabetes, or pituitary issues
- For testosterone testing in men: arrange the appointment for early morning, fasting
Related Reading
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