Effect of aminoglycosides on the disposition of thyroid hormones and thyroglobulin

Our study was designed to confirm the potential effects of three aminoglycosides on the disposition of thyroid hormones. Twenty-seven patients diagnosed with either cellulitis (n = 19), chronic osteitis (n = 4), or an abscess (n = 4) were selected. Thirteen patients received tobramycin, 60 to 100 mg iv q. 8 h., plus cloxacillin, 1 gm iv q. 4 h.; seven patients received netilmicin, 40 to 120 mg iv q. 8 h., plus cloxacillin, 1 gm iv q. 4 h.; and seven patients received either cloxacillin, 1.5 gm iv q. 4 h., or cefoperazone, 2 to 4 gm iv q. 12 h. for at least 7 days. Another group of six normal subjects received neomycin, 0.5 gm po q. 6 h. for 7 days. All these subjects had normal thyroid function before antibiotic dosing and none had thyroid function abnormalities. Tobramycin and cloxacillin/cefoperazone did not influence thyroid function. Netilmicin decreased the total serum concentrations of triiodothyronine (T3) from 114 +/- 9 to 75 +/- 7 ng/dl (P less than 0.01), probably because of increased clearance, as the T3 free fraction increased from 0.43% +/- 0.02% to 0.49% +/- 0.02% (P less than 0.05). Thyroxine (T4) and reverse T3 (rT3) levels were not affected. Neomycin decreased T3 levels from 104 +/- 8 to 92 +/- 7 ng/dl (P less than 0.05) and the serum concentrations of thyroglobulin from 17.3 +/- 2.0 to 11.7 +/- 2.0 ng/ml (P less than 0.001). Because T4 and rT3 levels did not change, our results suggest that neomycin may have directly affected the gland. We conclude that some aminoglycosides can alter the disposition of thyroid hormones.