Factors That Influence Levothyroxine Dose
Levothyroxine is the form of thyroid hormone that is generally
recommended for replacement or suppressive therapy. Once a patient
has been taking levothyroxine regularly for four to six weeks, measurement
of serum TSH can indicate whether the patient is taking the correct
dose. If the patient is taking too much levothyroxine, the pituitary
stops making TSH and the level falls; if the dose is too low, TSH
increases.
The dose of levothyroxine that an individual requires is dependent
partly upon body weight, age, and sex. Patients who weigh more require
higher doses, while elderly patients require considerably less hormone.
Women in general require less levothyroxine than men. Average replacement
doses for women are 0.112-0.125 mg daily, but may fall to 0.075
mg during the seventh and eighth decades. Men generally require
0.025-0.050 mg more levothyroxine than women.
Approximately three-fourths of the levothyroxine (Synthroid, Levothroid,
Levoxyl, and others) that a patient ingests is absorbed and finds
its way into the bloodstream. Since the introduction of sensitive
TSH assays which allow us to determine the appropriateness of an
individual’s levothyroxine dose more precisely than ever before,
a number of factors have been found which interfere with the absorption
of levothyroxine. This can either lead to variability of hormone
levels, or increased dose requirements. When a patient’s dose
requirements seem to fluctuate unexpectedly, one must consider factors
that can interfere with absorption.
The cholesterol lowering resins cholestyramine (Questran) and colestipol,
and the antacid medication carafate may bind levothyroxine in the
gastrointestinal tract and prevent its absorption. Aluminum-containing
antacids and iron tablets or iron-containing vitamins (common over-the-counter
medications), also can interfere with levothyroxine absorption.
And a recent report suggests that dietary bran might also bind and
limit the absorption of levothyroxine. It is likely that this list
of medications and foods will increase in the future.
For this reason, many pharmacists now recommend that levothyroxine
be taken on an empty stomach.
For patients who have taken levothyroxine for years, who now use
the medications listed in the prior paragraph, and who have had
stable TSH levels, I do not recommend a change in the manner or
time of day that they take their medication. Theoretically, if a
patient has always taken levothyroxine with breakfast and had a
normal TSH, taking it on an empty stomach might result in higher
absorption and require a subsequent reduction in dose. However,
patients who have had variable TSH levels or difficulty adjusting
the dose should take levothyroxine on an empty stomach—or
several hours after ingesting food or other medications. And certainly
if the need arises to take any of the medications noted above, they
should be taken several hours before or after taking levothyroxine.
There are several other medications and diseases that can influence
the dose of levothyroxine that an individual patient requires. Patients
with gastrointestinal disorders that result in malabsorption such
as chronic diarrhea, inflammatory bowel disease or sprue may require
very high doses of levothyroxine because of poor absorption. Patients
with nephrotic syndrome may lose thyroid hormone in their urine
and require higher doses of levothyroxine. And several drugs increase
the metabolism of levothyroxine and patients starting these medications
may require a higher dose: dilantin, carbamezine (Tegretol), phenobarbitol,
and rifampin.
Finally, pregnancy is associated with significantly increased levothyroxine
requirements and many hypothyroid pregnant women will require a
30% or higher increase in their dose. Estrogen therapy increases
the protein that binds thyroxine in the blood. It is possible that
some patients who start treatment with estrogen hormones (for example,
Premarin or oral contraceptives) may require an adjustment in their
levothyroxine dose. It is prudent to check a TSH a few months after
starting an estrogen containing medication to be sure that the dose
requirements have not changed.
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