Common causes shared by women and men
- Increased calorie intake, larger portions, sugary drinks or frequent restaurant food
- Reduced physical activity, prolonged sitting or loss of muscle mass
- Insufficient sleep, shift work or untreated sleep apnoea
- Stress, depression, binge-eating symptoms or emotional eating
- Medicines such as corticosteroids, some psychiatric medicines, some seizure medicines and selected diabetes treatments
- Age-related changes, family susceptibility and an environment that promotes high-calorie intake
Common considerations in women
PCOS—now also called PMOS
Polyendocrine metabolic ovarian syndrome (PMOS), previously called PCOS, is strongly linked with insulin resistance and may be accompanied by easier weight gain, irregular cycles, acne, facial hair or scalp thinning. PMOS can occur at any body size; weight is not required for the diagnosis.
Pregnancy, postpartum changes and menopause
Pregnancy-related weight retention, disrupted sleep, reduced activity and postpartum mood changes can all matter. During the menopause transition, changes in fat distribution, sleep and muscle mass may promote abdominal weight gain even when the scale changes only modestly.
Thyroid disease
Hypothyroidism can cause some weight gain, much of it from salt and water retention, but it usually does not explain severe obesity by itself. If thyroid tests are normal, increasing thyroid medicine is not a safe weight-loss strategy.
Common considerations in men
Sleep apnoea, alcohol intake, reduced activity, loss of muscle mass and weight-promoting medicines are common contributors. Low testosterone may coexist with obesity, but diagnosis requires compatible symptoms and properly timed laboratory testing; obesity itself can lower measured testosterone. Testosterone should not be used as a general weight-loss treatment without a confirmed indication.
Less common hormonal or medical causes
Cushing syndrome, hypothalamic disease and other endocrine disorders can cause weight gain but are far less common than sleep, medicines, diet, activity and ordinary metabolic susceptibility. Testing should be guided by specific clinical features rather than a broad “hormone panel.”
When weight gain deserves prompt medical review
- Rapid weight increase with leg swelling, breathlessness or abdominal swelling
- Marked thirst, frequent urination or blurred vision
- New purple stretch marks, easy bruising or pronounced muscle weakness
- Irregular cycles, increasing facial hair or scalp hair thinning
- Weight gain after starting a new medicine
What a medical assessment can clarify
Assessment may review the weight timeline, appetite, sleep, medicines, menstrual or sexual symptoms, family history and weight-related complications. Testing may include glucose or HbA1c, lipids, liver and kidney measures and thyroid tests when clinically indicated. Treatment is then matched to the dominant drivers and may include lifestyle care, medication review, management of related disease and obesity medication for appropriate patients.
Primary sources
- NIDDK: Factors affecting weight and health, reviewed May 2023.
- American Thyroid Association: Thyroid and weight, accessed July 2026.
- Monash University: PMOS/PCOS international guideline resources, updated May 2026.
This article is educational. Rapid unexplained weight gain or swelling may require urgent assessment. Do not stop a prescribed medicine without discussing it with the prescriber.