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Yankees icon Lou Gehrig struggled to perform simple tasks — like buttoning his shirt — only months after retirement, but was still convinced he’d beat ALS, according to a heartbreaking letter penned by the Iron Horse.

A Southern California auctioneer is hawking one of the final documents Gehrig ever signed, a Sept. 13, 1939, note to his Mayo Clinic physician, Dr. Paul O'Leary.

Gehrig told the doctor that the medication he had prescribed was already helping, as the Bombers first baseman was in the early stages of ALS — now better known as Lou Gehrig's Disease.

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The all-time Yankees great suddenly retired early in the 1939 season and died on June 2, 1941, at the age of 37.

"Where I used to get exceptionally tired in the morning [especially the right hand] from brushing my teeth, shaving, combing my hair, buttoning up tight buttons on my clothes, I would then feel like resting and relaxing, whereas now that tiredness is somewhat lessened, and I still have pep to go on," according to Gehrig's letter.

Gehrig believed he was being cured by injections of thiamine — vitamin B1. Sadly, the improvement Gehrig felt was just in his head.

"Lou Gehrig's perceived beneficial response to the injections was likely a placebo response," said Dr. Stephen Scelsa, director of the Neuromuscular Division and ALS Center at Mount Sinai Beth Israel.

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