CLINICAL PHARMACOLOGY
The mechanisms by which thyroid hormones exert their
physiologic action are not well understood. These hormones
enhance oxygen consumption by most tissues of the body
and increase the basal metabolic rate and the metabolism
of carbohydrates, lipids and proteins. Thus, they exert
a profound influence on every organ system in the body
and are of particular importance in the development of
the central nervous system.
Pharmacokinetics
Since liothyronine sodium (T3) is not firmly bound to
serum protein, it is readily available to body tissues.
The onset of activity of liothyronine sodium is rapid
occurring within a few hours. Maximum pharmacologic response
occurs within 2 or 3 days, providing early clinical response.
The biological half-life is about 2-½ days.
T3 is almost totally absorbed, 95 percent in 4 hours.
The hormones contained in the natural preparations are
absorbed in a manner similar to the synthetic hormones.
Liothyronine sodium has a rapid cutoff of activity which
permits quick dosage adjustment and facilitates control
of the effects of overdosage, should they occur.
The higher affinity of levothyroxine (T4) for both thyroid-binding
globulin and thyroid-binding prealbumin as compared to
triiodothyronine (T3) partially explains the higher serum
levels and longer half-life of the former hormone. Both
protein-bound hormones exist in reverse equilibrium with
minute amounts of free hormone, the latter accounting
for the metabolic activity.
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