Mood swings: what causes them, the timing that sorts them, and the tests that help

A mood swing is a fast shift in mood, from fine to snappy, tearful or flat, that feels bigger than whatever set it off. Before you order a single test, write down when your mood swings happen for two to four weeks. That diary sorts most people into one of a handful of patterns, and each pattern has its own short list of causes and its own right test, or no test at all.

Start with the pattern: when do your mood swings happen?

Sort your mood swings by timing first. A swing that lasts an hour after a skipped lunch and an episode of high mood lasting a week are different problems, even if both feel like losing control of your temper. Match your diary to the closest row below.

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Mood swings before your period: PMS or PMDD?

Premenstrual syndrome (PMS) is a set of mood and body symptoms in the luteal phase, the one to two weeks between ovulation and bleeding, that clear within a few days of the period starting. The American College of Obstetricians and Gynecologists (ACOG) puts PMS at roughly 20% to 32% of premenopausal women and PMDD, a severe form where irritability, low mood or anxiety disrupt work and relationships, at about 3% to 8% ¹.

PMDD is diagnosed by rating symptoms every day across at least two menstrual cycles, then checking that they rise before bleeding and fall after it ¹. There is no blood test for PMDD. Hormone levels in women with PMDD are typically normal; the brain appears to react more strongly to the normal rise and fall of estradiol (the main estrogen) and progesterone ¹. A 'hormone imbalance' panel will usually come back in range and tell you nothing about PMS or PMDD. How the cycle hormones move is covered in the menstrual cycle hormones guide.

ACOG lists regular exercise, cognitive behavioral therapy, calcium, SSRIs (antidepressants a doctor prescribes, sometimes only for the luteal half of the cycle) and combined hormonal contraception as options for premenstrual symptoms ¹. Ask your doctor which suits you; calcium explains the supplement side. If your cycles are also irregular or your period goes missing, or you have acne and extra hair growth, ask about PCOS, which shows up on blood tests in a way PMS does not.

Perimenopause, pregnancy and after a baby

In perimenopause, the years before periods stop, estradiol swings higher and lower than in earlier cycles. A US study that followed women through the menopause transition found low mood was more likely during it than before, linked to these changing hormones ⁷. UK guidance (NICE NG23) advises diagnosing perimenopause from symptoms in women over 45, without an FSH test, because FSH bounces around from month to month ⁸. If symptoms are disrupting life, ask about hormone therapy; HRT explains the options. Read more on perimenopause, menopause, and the menopause hub.

After a baby, the baby blues (tearfulness and mood swings in the first days) affect most new mothers and fade within about two weeks. Low mood, anxiety or irritability that lasts past two weeks, or that makes it hard to care for yourself or the baby, can be postpartum depression and needs a doctor or midwife. Confusion, hallucinations or very little sleep with high energy in the weeks after birth can be postpartum psychosis, which is an emergency: call 911 or go to the emergency department.

Mood swings in men: testosterone and more

Low testosterone is linked with irritability, low mood and low drive, but those symptoms are common and have many causes. The Endocrine Society guideline says low testosterone should be diagnosed only when consistent symptoms come with unequivocally low total testosterone on a fasting morning sample, confirmed on a repeat test ³. SHBG, the protein that carries testosterone in blood, can make a total testosterone result misleading when it runs high or low. See low testosterone, total testosterone, SHBG and the male hormone panel explained.

CalculatorFree testosterone calculatorHave total testosterone, SHBG and albumin on your report? Estimate the free testosterone that a low-T diagnosis often turns on.Work it out

In men, the other common drivers of mood swings are alcohol (and the irritable morning after), untreated sleep apnea (loud snoring, gasping, daytime sleepiness), anabolic steroids, which raise irritability on a cycle and can drop mood sharply after one, and ordinary work and money stress.

Thyroid: when it runs too fast or too slow

An overactive thyroid tends to bring anxiety, irritability, a racing heart and weight loss; an underactive one tends to bring low mood, slowness, feeling cold and weight gain. Less often, early Hashimoto's disease passes through a brief overactive phase before it settles low. NICE guideline NG145 suggests considering thyroid function tests in adults with depression or unexplained anxiety ². The two conditions side by side are in hypothyroidism vs hyperthyroidism.

TSH comes first, with free T4 (the thyroid hormone itself) added if TSH is off, and TPO (thyroid peroxidase) antibodies, which show whether the immune system is attacking the thyroid. Reading the numbers is in how to read your thyroid panel; if your report also shows T3, see T3 vs T4. High-dose biotin supplements can distort thyroid results, as biotin and blood tests explains.

Low B12, iron or vitamin D

Vitamin B12 deficiency can cause irritability, low mood, poor memory and numbness or tingling. In a New England Journal of Medicine series, 28% of patients with nerve or psychiatric problems from B12 deficiency had no anemia, so a normal blood count does not rule low B12 out ⁶. If B12 is low-normal and you have tingling or memory trouble, a doctor may check methylmalonic acid (MMA). Numbness, pins and needles or an unsteady walk alongside mood swings need a doctor soon, because B12-related nerve damage can become permanent if it is left. More in B12 deficiency and pernicious anemia.

Low iron stores cause tiredness, poor concentration and a short fuse, sometimes before hemoglobin falls. As ferritin vs iron puts it: "Ferritin estimates the iron held in storage and changes over weeks." WHO guidance uses a ferritin below 15 µg/L to indicate iron deficiency in healthy adults ¹². Do not start iron on a hunch: too much iron is harmful, and the cause of low iron (often heavy periods or gut blood loss) matters as much as the number. See iron deficiency.

Low vitamin D often shows up alongside depression, but the largest trial tested whether supplements prevent it. In VITAL-DEP, more than 18,000 adults aged 50 and over took vitamin D3 or placebo for about five years, and vitamin D did not lower the risk of depression or improve mood scores ⁴. A real vitamin D deficiency is worth treating with your doctor for bones and muscles, but the trial evidence says not to expect it to fix mood swings.

These findings do not support the use of vitamin D3 in adults to prevent depression.Okereke et al., VITAL-DEP, JAMA 2020 ⁴

Inflammation matters too. Long-term inflammatory and autoimmune conditions such as lupus, celiac disease and inflammatory bowel disease often come with low mood, irritability and fatigue. A CRP test is worth checking when there are other signs, such as joint pain, rashes, diarrhea or weight loss; it does not diagnose a mood problem. Chronic inflammation covers the wider picture.

Blood sugar, hunger and caffeine

Being 'hangry' has a mechanism. When blood glucose falls, the body releases adrenaline to push it back up, and adrenaline brings shakiness, sweating, a fast heart and irritability. The American Diabetes Association defines level 1 hypoglycemia as glucose below 70 mg/dL (3.9 mmol/L) and level 2 as below 54 mg/dL (3.0 mmol/L) ⁹; those levels were set for people with diabetes. True low glucose is uncommon in people who do not take diabetes medicines; most hunger irritability happens at normal glucose and eases with regular meals. See hypoglycemia, insulin resistance and supplements for blood sugar.

Caffeine works on mood from the other side. A review of caffeine withdrawal studies found that stopping caffeine can cause irritability, low mood, headache and fatigue, usually starting 12 to 24 hours after the last dose ¹⁰. A person who drinks four coffees on workdays and none at weekends can get a Saturday mood dip that looks like a mood problem.

Sleep, stress, alcohol and medicines

After one night without sleep, the amygdala, the brain area that reacts to threat, responded about 60% more strongly to upsetting images than in rested people ⁵. That stronger reaction is why a short night so often ends in a short fuse, and mood swings in turn make sleep worse. If sleep is the problem, start with why you can't sleep. Alcohol lifts mood for an hour and lowers it the next day, fragments sleep, and causes anxiety and irritability during withdrawal in heavy drinkers.

Several medicines shift mood. Steroid tablets such as prednisone can cause mood changes, irritability and, at high doses, more serious psychiatric reactions, and the risk rises with the dose. Hormonal contraception, isotretinoin for acne, anabolic steroids and stimulants are other common culprits. A mood change that started within weeks of a new medicine is worth raising with the prescriber.

Stress raises cortisol, but a single blood cortisol cannot measure stress, because cortisol peaks in the morning and changes hour to hour. A systematic review found no evidence that 'adrenal fatigue' exists as a medical condition ¹³. As adrenal fatigue puts it: "The exhaustion is real; the worn-out-gland explanation is not." Morning cortisol is useful when a doctor is looking for a real adrenal disease such as Cushing's syndrome, which is rare and comes with weight gain, easy bruising and high blood pressure. Practical steps are in how to lower cortisol.

When mood swings are a mental health condition

Duration separates ordinary mood swings from bipolar disorder. DSM-5-TR, the American Psychiatric Association's diagnostic manual, requires a manic episode to last at least 7 days (or any length if it leads to hospital) and a hypomanic episode at least 4 days in a row, with raised energy and less need for sleep ¹¹. Shifts from hour to hour do not meet those criteria on their own. Cyclothymia is a milder pattern of many highs and lows over at least two years.

Other conditions cause shorter swings. Mood shifts that last hours and follow conflict or fear of rejection can fit borderline personality disorder; quick frustration and outbursts can come with ADHD; and depression can show up mainly as irritability, which some men notice more than sadness. Blood tests cannot diagnose any of these conditions. Their job is to rule out thyroid disease, low B12, anemia or a medicine as the cause, so a doctor or psychiatrist can assess the rest with a clear picture.

Which blood tests help (and which don't)

A focused first panel covers the common physical causes of mood swings. Other tests earn their place only in a specific situation. What a blood test can detect covers the limits more broadly.

Testosterone needs a fasting sample taken before about 10 am, and a progesterone result is most useful when taken about seven days before the next period. If you take high-dose biotin, ask whoever orders your thyroid test whether to pause it for a few days first; supplements that affect blood tests lists others. The rest of the preparation is in how to prepare for a blood test, and the women's panel is explained in the female hormone panel guide.

An all-normal first panel is useful information. Normal TSH, CBC, ferritin, B12 and glucose make the common physical causes of mood swings much less likely, which points you and your doctor back to the timing pattern: sleep, the cycle, alcohol, caffeine, medicines and mental health.

What may help this week

1. 1Keep a mood diaryEach evening, rate mood from 1 to 10 and note sleep hours, cycle day, alcohol, caffeine, meals and any new medicine. Keep it for 4 to 8 weeks, or two full cycles if you menstruate.
2. 2Eat at regular timesLong gaps between meals make hunger irritability more likely; meals with protein and fiber may soften the dip.
3. 3Hold caffeine steadyKeep the amount the same every day, weekends included, and cut back slowly if you want to cut back at all.
4. 4Protect sleepKeep a fixed wake time, sleep in a dark, cool room, and avoid alcohol in the 3 hours before bed. Even one short night can make mood more reactive the next day.
5. 5Move most daysRegular exercise is on ACOG's list for premenstrual symptoms and can help low mood generally.
6. 6Take the diary to a doctorA diary that shows when mood swings happen gets you to the right test, or the right referral, in one visit.

When to see a doctor, and when it's urgent

Book a routine appointment if mood swings:

• last more than two weeks or keep coming back
• affect work, study or relationships
• come with weight change, a racing heart, heavy periods, tingling or constant tiredness
• started after a new medicine or a change of dose
• make you lean on alcohol or other drugs to cope

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