DESCRIPTION
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-triiodothyronine or LT3), a synthetic form of a
natural thyroid hormone, and is available as the sodium salt.
The structural and empirical formulas and molecular weight of liothyronine
sodium are given below.
Liothyronine Sodium
L-Tyrosine, O-(4-hydroxy-3-iodophenyl)-3,5-diiodo-, monosodium salt
Twenty-five mcg of liothyronine is equivalent to approximately 1 grain 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 KPI
and 115; 25 mcg scored and debossed KPI and 116; 50 mcg scored and debossed KPI
and 117. Inactive ingredients consist of calcium sulfate, gelatin, starch,
stearic acid, sucrose and talc.
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