Cytomel is a prescription medication used to treat the symptoms of low thyroid hormone levels (hypothyroidism) and enlarged thyroid gland (nontoxic goiter), myxedema, and myxedema coma. Cytomel may be used alone or with other medications. Cytomel belongs to a class of medications called thyroid products. Thyroid hormone medications are natural or synthetic preparations containing tetraiodothyronine (T4, levothyroxine) sodium or triiodothyronine (T3, liothyronine) sodium, or both. T4 and T3 are produced in the human thyroid gland by iodination and coupling of the amino acid tyrosine. T4 contains four iodine atoms and is formed by the coupling of two diiodotyrosine (DIT) molecules. T3 contains three iodine atoms and is formed by the coupling of one DIT molecule with one monoiodotyrosine (MIT) molecule. Both hormones are stored in colloidal thyroid thyroglobulin. Thyroid hormone preparations fall into two categories: natural hormone preparations derived from animal thyroid and synthetic preparations. Natural preparations include desiccated thyroid and thyroglobulin. Desiccated thyroid is derived from domesticated animals that man uses for food (either beef or pig thyroid), and thyroglobulin is derived from pig thyroid glands. The United States Pharmacopeia (USP) has standardized the total iodine content of mature preparations. Thyroid USP contains not less than (NLT) 0.17 percent and not more than (NMT) 0.23 percent iodine, and thyroglobulin contains not less than (Not Less than) 0.7 percent organically bound iodine. Sludge content is only an indirect measure.