The dosage requirement for levothyroxine replacement therapy may be affected by concurrent diseases. Identification of these factors can help identify a patient who may be over or under treated, requiring an adjustment in the daily dose.


Factors requiring a decrease in dose:

(1) advancing age (>= 65 years of age)

(2) androgen therapy (in women)


Factors requiring an increase in dose:

(1) drugs decreasing GI absorption (see below)

(2) drugs causing an increase in cytochrome P450 activity (see below)

(3) gastrointestinal disease (see below)

(4) pregnancy

(5) selenium deficiency (blocks deiodinase synthesis)

(6) cirrhosis (blocks deiodinase synthesis)

(7) amiodarone (blocks conversion of T4 to T3)


Drugs resulting in decreased absorption:

(1) cholestyramine

(2) sucralfate

(3) aluminum hydroxide

(4) calcium carbonate

(5) ferrous sulfate

(6) omeprazole and other drugs suppressing gastric acid production


Drugs increasing cytochrome P450 (CYP3A4) activity:

(1) estrogen

(2) carbamazepine

(3) rifampin

(4) phenytoin

(5) sertraline


Drugs reducing gastrointestinal absorption:

(1) atrophic gastritis

(2) Helicobacter pylori gastritis

(3) sprue, Giardiasis and other causes of malabsorption

(4) diabetic diarrhea

(5) after small bowel resection (short bowel syndrome)

(6) after jejunoileal bypass


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