The invention provides a method for treating amyotrophic lateral sclerosis (ALS) in a subject. The method comprises administering to the nervous system of the subject a composition comprising a thyroxine protein or a therapeutic fragment or pharmacologic mimic thereof and a pharmaceutically acceptable carrier. The invention also provides a method for treating ALS in a subject that comprises administering to the subject a transthyretin protein, 7B2 protein, a cystatin C protein, a neuroendocrine protein, a cysteine protease inhibitor, or an inhibitor of an enzyme that processes a 7B2 protein. In addition, the invention provides methods for determining the susceptibility of a subject to developing ALS and for determining the progression of ALS in a subject.