Short answer: libido is the output of several systems at once — sex hormones, thyroid, sleep, mood, medication, physical health and the relationship it exists in. When desire drops, one or more of those has usually moved. The useful question is not "how do I want sex more" but "which of these has changed".
What "normal" actually means
There is no correct amount of desire. Libido varies enormously between people and across a lifetime, and a mismatch within a couple is not the same as a disorder in either person. The thing worth paying attention to is a change from your own baseline — particularly one that is persistent, unexplained, and bothering you.
That last clause matters. Low desire that causes you no distress is not a medical problem.
Hormonal causes
Testosterone
Testosterone drives desire in both sexes, though at very different concentrations. In men it declines gradually from roughly the age of 30, and genuinely low levels can flatten libido, energy, mood and muscle mass together. In women it also contributes to desire, and falls across the menopausal transition.
Symptoms alone cannot tell you whether yours is low — fatigue and low desire have dozens of causes. That is what a blood test is for.
Oestrogen and the menopause transition
Falling oestrogen affects desire directly and indirectly: it changes arousal and lubrication, and it disrupts sleep and mood via hot flushes and night sweats. See menopause, perimenopause and sex for the detail.
Thyroid
An underactive thyroid is a classic mimic — fatigue, weight gain, low mood, low libido — and it is common, easily measured and very treatable. It is one of the more satisfying things to rule in or out early.
Prolactin
Raised prolactin suppresses sex drive and is worth checking when libido has fallen sharply without obvious cause. It is included in the male and female hormone panels.
Measure rather than guess
Testosterone, thyroid, prolactin and full male or female hormone profiles, analysed by a UKAS-accredited UK laboratory. Morning sample, discreet packaging, your numbers against standard reference ranges.
See hormone testsMedication — the cause people miss
A great many routine medicines reduce libido, and the effect is often not mentioned when they are prescribed. The common culprits include SSRI and SNRI antidepressants, some blood pressure medications including beta-blockers, hormonal contraception, finasteride, and opioid painkillers.
Do not stop a prescribed medicine on your own. Take it to the prescriber — there is frequently an alternative within the same class, or a dose adjustment, that suits you better.
Sleep, stress and the physiology of not being in the mood
Desire is expensive. It is one of the first things the body deprioritises under sustained stress or sleep debt, and that is a sensible adaptation rather than a malfunction. Chronic short sleep lowers testosterone in men measurably within a week, and elevated cortisol suppresses sex hormones in both sexes.
If you are averaging five hours and running on adrenaline, low libido is not a mystery to solve. It is a symptom that will lift when the cause does.
Mental health and relationships
Depression reduces desire directly, and its treatments frequently reduce it further — a genuinely difficult trade-off worth discussing openly with a GP rather than quietly abandoning treatment over.
Relational causes are equally real and frequently harder to say out loud: unresolved resentment, mismatched expectations, loss of novelty, or sex having become a source of pressure rather than pleasure. Where that is the driver, no hormone panel will help, and psychosexual therapy often will. Relate and the College of Sexual and Relationship Therapists both list UK practitioners.
Working out which one is yours
- Did it change suddenly or gradually? Sudden changes point toward medication, illness, or a life event. Gradual ones point toward hormones or accumulated stress.
- Is it global or situational? No desire in any context suggests a physiological or mood cause. Desire that exists alone or with a different partner points elsewhere.
- What else changed? New medication, poor sleep, weight change, a new stressor, or other symptoms such as fatigue and low mood all narrow it considerably.
- Get the measurable things measured. Thyroid, testosterone, prolactin and iron are cheap to check and pointless to speculate about.
libido has dropped sharply without explanation, you have other symptoms such as fatigue, low mood, hair loss or unexplained weight change, you suspect a prescribed medicine is responsible, or low desire is accompanied by erectile difficulties — which can be an early cardiovascular sign.
Common questions
Is low libido a normal part of ageing?
Some decline is typical, but a marked or sudden drop is not simply age and is worth investigating. Testosterone falls gradually in men and oestrogen falls sharply around menopause in women, but fatigue, medication, thyroid problems and stress are just as common and considerably more treatable.
Can low testosterone be the cause of low sex drive?
It can be, in both men and women, but it is one cause among many and cannot be diagnosed from symptoms. Testosterone follows a daily rhythm and is highest in the morning, so a morning sample is needed for a meaningful result.
Which blood test should I get for low libido?
For men, a testosterone or full male hormone panel. For women, a female hormone panel. Both cover the hormones most likely to be involved. If you also have fatigue or weight change, thyroid markers are worth including.
Can antidepressants cause low libido?
Yes — SSRIs and SNRIs commonly reduce desire, arousal and orgasm. Never stop a prescribed antidepressant on your own; speak to the prescriber, as alternatives and dose adjustments exist.
How long should I wait before seeing someone about it?
If it has persisted for several months, is causing you distress, or came with other symptoms, that is reason enough. There is no threshold you need to pass first.
Will a blood test tell me why my libido is low?
It will rule several common causes in or out — testosterone, thyroid, prolactin — which is genuinely useful. It cannot assess stress, mood, relationship factors or medication effects, so it narrows the question rather than answering it outright.