Assessment of interchangeability of two brands of levothyroxine preparations with a third-generation TSH assay.

PURPOSE: To evaluate, in a prospective fashion, the clinical interchangeability between two brands of levothyroxine, Synthroid (Boots Pharmaceuticals, Inc., Lincolnshire, Illinois) and Levoxine (Daniels Pharmaceuticals, Inc., St. Petersburg, Florida), by using clinical scores of hyperthyroidism and hypothyroidism, free thyroxine index (FTI), sensitive thyroid-stimulating hormone (TSH), and thyrotropin-releasing hormone (TRH) stimulation testing. PATIENTS AND METHODS: Twenty-three of the 31 patients with long-standing primary hypothyroidism (6 men, 25 women; age range 30 to 71 years, mean 47.2 +/- 2.2 SEM) were switched from Synthroid to Levoxine (group 1) and the remaining patients from Levoxine to Synthroid (group 2). After switching, each patient continued to receive the same dosage as previously. Clinical scores of hypothyroidism and hyperthyroidism (Billewicz and Crooks scoring systems, respectively), basal FTI, and TRH stimulation test were obtained before and 4 months after the switching. Comparison of the variables before and after switching was performed separately in each subgroup and in the entire group. RESULTS: There was no statistically significant difference in the hypothyroid clinical scores (-40.1 +/- 1.2 versus -39.7 +/- 1.2), the hyperthyroid clinical scores (-19.6 +/- 0.9 versus -19.2 +/- 1.0), FTI (9.6 +/- 0.3 versus 9.6 +/- 0.3), basal TSH levels (1.4 +/- 0.2 versus 1.4 +/- 0.2 mIU/L), or the magnitude of TSH response to TRH (mean delta TSH 9.4 +/- 1.5 versus 9.2 +/- 1.4 mIU/L), whether the patients were receiving Synthroid or Levoxine. CONCLUSIONS: Switching did not result in substantial clinical or laboratory changes in any individual patient. We conclude that the two brands of levothyroxine are clinically interchangeable.

Effect of L-thyroxine therapy on lipoprotein fractions in overt and subclinical hypothyroidism, with special reference to lipoprotein(a).

The effect of L-thyroxine therapy on lipoprotein fractions was assessed in 15 patients with overt hypothyroidism (14 women and one man aged 45 +/- 3.9 years; thyrotropin [TSH]: mean +/- SEM, 42 +/- 6.5 mIU/L; range, 20.5 to 106.5) and 14 patients with subclinical hypothyroidism (13 women and one man aged 41 +/- 4 years; TSH: mean +/- SEM, 9.1 +/- 1 mIU/L ; range 5.1 to 17.3). Fasting serum lipid levels were measured initially and 4 months after achievement of a euthyroid state with incremental L-thyroxine therapy (TSH: mean +/- SEM, 1.8 +/- 0.4 mIU/L; range, 0.3 to 4.9 for both groups). In the overtly hypothyroid group, restoration of a euthyroid state was associated with a significant reduction in total cholesterol, and apo B. In the subclinically hypothyroid group, there was a significant reduction of only total cholesterol (199.6 +/- 13.2 v 183.4 +/- 11.6 mg/dL) and LDL-C (13.6 +/- 8.4 v 114 +/- 9.25 mg/dL). In contrast, lipoprotein(a) [Lp(a)] was unaffected by the incremental adjustment of L-thyroxine therapy in both groups (overt, 34.3 +/- 8.8 v 35.6 +/- 6.7 mg/dL; subclinical, 23.0 +/- 8.6 v 29.4 +/- 9.5 mg/dL). We conclude that restoration of a euthyroid state in patients with overt hypothyroidism has no significant effect on Lp(a) levels, and confirm that subclinical hypothyroidism is associated with a significant increase in LDL-C, known to have an atherogenic effect.