When is weight gain a medical problem rather than a calorie problem?

Most weight gain is not caused by a single hidden diagnosis. Some of it is, and the pattern usually gives it away. Rapid gain, gain that started at an identifiable moment, gain with other symptoms, or gain in someone whose eating has genuinely not changed all justify looking properly before anyone talks about calories.

The medications that cause weight gain

This is the commonest medical cause and the most frequently missed. The 2015 Endocrine Society guideline on the pharmacological management of obesity recommends selecting weight-neutral alternatives wherever clinically feasible, and names the categories. Several antipsychotics cause substantial gain. So do some antidepressants and antiepileptics. Insulin and sulfonylureas promote gain in type 2 diabetes, which is why the guideline recommends adding metformin or a GLP-1 where insulin is needed. Beta blockers are associated with gain, and the guideline prefers ACE inhibitors, angiotensin receptor blockers or calcium channel blockers as first-line antihypertensives in this population. Corticosteroids, injectable contraceptives and sedating antihistamines all appear on the list.

The endocrine causes worth excluding

The Endocrine Society advises screening for secondary causes when the history or examination suggests them. Hypothyroidism is the most commonly tested and typically produces a modest gain, largely fluid, that reverses with treatment. Cushing syndrome is rare but should be considered with central weight gain, purple stretch marks, easy bruising, proximal muscle weakness, new diabetes and new hypertension appearing together. Growth hormone deficiency, hypothalamic damage after surgery or radiotherapy, and pseudohypoparathyroidism are all in the differential. Polycystic ovary syndrome does not usually cause abrupt weight gain but does make loss harder through insulin resistance.

Sleep and hormonal transitions

Obstructive sleep apnoea and weight form a loop: the apnoea worsens with weight and drives further weight gain through disturbed sleep, appetite dysregulation and daytime fatigue. It is treatable, and treating it changes the trajectory. Tirzepatide is now approved for moderate to severe obstructive sleep apnoea in adults with obesity, having reduced the apnoea-hypopnoea index by about 25 to 29 events per hour in the SURMOUNT-OSA trials. Perimenopause shifts fat distribution towards the abdomen and reduces insulin sensitivity, which is why the same eating pattern produces a different body after 45.

Bring three things to the appointment where this is assessed: a timeline of when the weight changed, a complete medication list including anything started in the six months before the change, and a note of any other new symptom, however unrelated it seems. That is what turns an assumption about calories into a diagnosis.