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Subject: [THYROID] Absorption of T4 supplementation
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Pharmacokinetics of Levothyroxine (T4):

Few clinical studies have evaluated the kinetics of orally administered
thyroid hormone. In animals, the most active sites of absorption appear to
be the proximal and mid-jejunum.

T4 is not absorbed from the stomach and little, if any, drug is absorbed
from the duodenum. There seems to be no absorption of the T4 from the
distal colon in animals.

A number of human studies have confirmed the importance of an intact
jejunum and ileum for T4 absorption and have shown some absorption from the
duodenum. Studies involving radioiodinated T4 fecal tracer excretion
methods, equilibration, and AUC methods have shown that absorption varies
from 48 to 80 percent of the administered dose.

The extent of absorption is increased in the fasting state and decreased in
malabsorption syndromes, such as sprue. Absorption may also decrease with
age.

The degree of T4 absorption is dependent on the product formulation as well
as on the character of the intestinal contents, including plasma protein
and soluble dietary factors, which bind thyroid hormone making it
unavailable for diffusion.

Decreased absorption may result from administration of infant soybean
formula, ferrous sulfate, sodium polystyrene sulfonate, aluminum hydroxide
sucralfate, or bile acid sequestrants.

From:  http://www.rxlist.com/cgi/generic/levothy.htm#sect-Description

Joan McPhee, not an M.D.
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