Weight is the output of several systems, and any one can drift. The list a doctor works from is short and treatable.

Thyroid running slow. Low thyroid hormone drops your resting energy burn and adds fluid; women and older adults are most at risk. One cheap blood test, the TSH, settles it ¹².

Sleep apnea behind the snoring. An airway collapsing dozens of times a night wrecks sleep and flips your appetite hormones; four in five cases go undiagnosed ³⁴.

A medication nobody added up. Whole drug classes push the scale up: some antipsychotics (olanzapine averages four kilograms in ten weeks), several antidepressants, steroids like prednisone, insulin, older beta-blockers, and lithium ⁵.

A body fighting regain. Anyone who lost real weight in the last decade carries a resting burn below what their size predicts and hunger hormones turned up. Six years after a televised weight-loss contest, entrants still burned five hundred fewer kilocalories a day than expected ⁶.

The rest of the list: polycystic ovary syndrome ⁷; menopause, which moves fat to the belly without much scale change ⁸; a steroid-driven Cushing's pattern ⁹; and short sleep on top of any of it ¹⁰.

Treating the cause returns energy, sleep, and mood before it returns weight. And when the workup finds nothing, an honest food log surfaces drift you missed ¹¹.

Walk into a primary-care visit with an organised ask, not an open-ended complaint. The workup is cheap and finite.

The arc, once the cause has a name:

• Weeks: cold hands warm, the afternoon wall goes, the fog lifts, usually before the scale moves ¹².
• Months: a defined treatment, the trousers fit again, the partner back in bed ³.
• A year or two: the blood pressure and HbA1c sliding the wrong way stop ¹³.