Hypothyroidism usually presents insidiously with symptoms such as fatigue, cold intolerance, and weight gain. Less common findings include myalgia, arthralgia, and joint effusion. In the patient described here, a triathlete, interpretation of early signs and symptoms as typical tendinitis led to months of treatment failure. Considering hypothyroidism in the differential diagnosis for patients who have overuse syndromes can expedite treatment. Definitive diagnosis rests on testing of serum thyroid hormone levels. Treatment, which is usually quickly effective, consists of gradually adjusted thyroid hormone replacement.