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
Stoffer SS, Szpunar WE. Levothyroxine Loses Potency With Age. JAMA. 1986;255(14):1881-1882. doi:10.1001/jama.1986.03370140079024