The Thyroid gland is the largest endocrine gland that affects all metabolic reactions in body because of its secretions. According to statistical evaluations the major dysfunction in thyroid is hypothyroidism that usually treated with levothyroxine. In this book at first the precise function of this gland is described whit simple slang and then evaluation of drug control is discussed. Understanding of the text needs no professional specialty and can be extended to other thyroid hormone replacement therapy. This book is recommended to everybody who wants to know monitoring of drug in this field, physicians, patients and all those interested.