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April 11, 1986

Levothyroxine Loses Potency With Age

Author Affiliations

Oakland Internists Associates Southfield, Mich

JAMA. 1986;255(14):1881-1882. doi:10.1001/jama.1986.03370140079024

To the Editor.—  We have reported in the past on variations in the potency of levothyroxine products. We initially reported that a brand name levothyroxine product (Synthroid) had only 78% of expected potency.1 We later reported that as a result of improved monitoring and possible changes in product manufacture, this product was more potent and had 100% of expected potency.2 Recently, we encountered difficulties in two patients who previously purchased Synthroid in bottles of 1,000 tablets.

Report of Cases.—  Case 1.—A 13-year-old boy with primary hypothyroidism was taking 0.15-mg tablets of Synthroid daily. He was generally compliant but would occasionally forget to take his medication two to three times per month. His mother had bought a bottle of 1,000 tablets of Synthroid in 1983. When tested in March 1985, his serum free thyroxine index (FTI) was 1.5 (normal, 1.4 to 4.0) and the serum thyroid-stimulating hormone (TSH) value