Evidence is presented that many people have hypothyroidism undetected by conventional laboratory thyroid-function tests, and cases are reported to support the empirical use of Armour thyroid. Clinical evaluation can identify individuals with sub-laboratory hypothyroidism who are likely to benefit from thyroid-replacement therapy. In a significant proportion of cases, treatment with thyroid hormone has resulted in marked improvement in chronic symptoms that had failed to respond to a wide array of conventional and alternative treatments. In some cases, treatment with desiccated thyroid has produced better clinical results than levothyroxine. Research supporting the existence of sub-laboratory hypothyroidism is reviewed, and the author's clinical approach to the diagnosis and treatment of this condition is described.