The serum is somehow reversed (Hydra weapon, Loki, Dr Doom, naturally wears off - whatever plot device works) and Steve is stuck in his pre-serum body, all his myriad disabilities included.

Then Hydra captures him. (Or Hydra captured him first, then undid the serum once they'd run all their tests and taken his blood)

Cue standard Hydra trash party, with pre-serum Steve dealing with no hearing in his left ear, heart palpitations + arrhythmia + angina (chest pain), not being able to see properly (astigmatism + color blindness), scoliosis, anemia, and did I mention the asthma?

And then Steve has a severe asthma attack mid-rape. And heart trouble. And just general skinny-Steve problems.

Bonus: still brainwashed Winter Soldier (so pre-Insight launch, but otherwise any point in the timeline you want) rescues him, vaguely recognizing that protecting skinny Steve is a mission that precedes and overrides all others. (He still thinks of it as a mission - clearly he's on Steve's protection detail). Of course after Hydra's had their fun