Usefulness of L-carnitine, a naturally occurring peripheral antagonist of thyroid hormone action, in iatrogenic hyperthyroidism: A randomized. double-blind, placebo-controlled clinical trial

Old studies in animals and unblinded studies in a few hyperthyroid patients suggested that L-carnitine is a periferal antagonist of thyroid hormone ac tion at least in some tissues. This conclusion was substantiated by our rec ent observation that carnitine inhibits thyroid hormone entry into the nucl eus of hepatocytes, neurons, and fibroblasts. In the randomized, double-bli nd, placebo-controlled 6-month trial reported here, we assessed whether 2 o r 4 g/d oral L-carnitine were able to both reverse and prevent/minimize nin e hyperthyroidism-related symptoms. We also evaluated changes on nine thyro id hormone-sensitive biochemical parameters and on vertebral and hip minera l density (bone mineral density). Fifty women under a fixed TSH-suppressive dose of L-T-4 for all 6 months were randomly allocated to five groups of 1 0 subjects each. Group 0 associated placebo for 6 months; groups A2 and A4 started associating placebo (first bimester), substituted placebo with 2 or 4 g/d carnitine (second bimester), and then returned to the association wi th placebo. Groups B2 and B4 started associating 2 and 4 g/d carnitine for the first two bimesters, and then substituted carnitine with placebo (third bimester). Symptoms and biochemical parameters worsened in group 0. In group A, sympto ms and biochemical parameters worsened during the first bimester, returned to baseline or increased minimally during the second bimester (except osteo calcin and urinary OH-proline), and worsened again in the third bimester. I n group B, symptoms and biochemical parameters (except osteocalcin and urin ary OH-proline) did not worsen or even improved over the first 4 months; th ey tended to worsen in the third bimester. In both the A and B groups, the two doses of carnitine were similarly effective. At the end of the trial, b one mineral density tended to increase in groups B and A (B > A). In conclusion, L-carnitine is effective in both reversing and preventing sy mptoms of hyperthyroidism and has a benefical effect on bone mineralization . Because hyperthyroidism. depletes the body deposits of carnitine and sinc e carnitine has no toxicity, teratogenicity, contraindications and interact ions with drugs, carnitine can be of clinical use.