Hypothyroidism is a disorder characterized by a deficiency of thyroid hormone and is not well understood in kittens. The condition is associated with failure to grow, mental dullness, constipation, and death. Treatment involves thyroid hormone supplementation which is readily available and inexpensive. Due to diagnosing hypothyroidism in a number of cases where orphan kittens failed to thrive, researchers have indicated this condition may be more common than previously known. Their goal is to establish normal reference intervals for thyroid hormone in kittens at 4 and 8 weeks of age, and to establish treatment guidelines for hypothyroidism in kittens.