Thyroid hormone drugs 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 the iodination and coupling
of the amino acid tyrosine T4 contains four iodine atoms and
is formed by the coupling of two molecules of diiodotyrosine (DIT). T3
contains three atoms of iodine and is formed by the coupling of one molecule
of DIT with one molecule of monoiodotyrosine (MIT). Both hormones are
stored in the thyroid colloid as thyroglobulin.
Thyroid hormone preparations belong to two categories (1) natural hormonal
preparations derived from animal thyroid, and (2) synthetic preparations.
Natural preparations include desiccated thyroid and thyroglobulin. Desiccated
thyroid is derived from domesticated animals that are used for food by
man (either beef or hog thyroid) and thyroglobulin is derived from thyroid
glands of the hog. The United States Pharmacopeia (USP) has standardized
the total iodine content of natural 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 (NLT) 0.7 percent of
organically bound iodine. Iodine content is only an indirect indicator
of true hormonal biologic activity.
Cytomel (liothyronine sodium) Tablets contain liothyronine (L-triiodothryonine
or LT3), a synthetic form of a natural thyroid hormone, and
is available as the sodium salt.
The empirical formula and molecular weight of liothyronine sodium are
|| M.W. 672.96
L-Tyrosine, O-(4-hydroxy-3-iodophenyl)-3,5-diiodo monosodium salt
Twenty-five mcg of liothyronine is equivalent to approximately 1 gram
of desiccated thyroid or thyroglobulin and 0.1 mg of L-thyroxine.
Each round, white to off-white Cytomel (liothyronine sodium) tablet contains
liothyronine sodium equivalent to liothyronine as follows:
5 mcg debossed SKF and D14; 25 mcg scored and debossed SKF and D16; 50
mcg scored and debossed SKF and D17. Inactive ingredients consist of calcium
sulfate, gelatin, starch, stearic acid, sucrose and talc.