Changes in thyroid status in pregnant women treated with ritodrine

In a clinical study of 17 pregnant women treated with ritodrine, a beta 2-sympathomimetic agent used for tocolysis, thyroid hormone status was assessed longitudinally. This was done in order to verify the hypothesis that an increase in T3 levels could result from adrenergic stimulation, since propranolol, a beta blocking agent, has proved to decrease T3 levels in man. Indeed, a statistically significant increase in T3 serum concentration and in T3/T4 ratio was found on the second day after the start of treatment with ritodrine (p less than 0.02 and less than 0.01 respectively). After discontinuation of treatment a decrease in T3 serum levels, compared to both treatment and pretreatment levels, was observed. The free T4 concentration showed a significant drop after the first week of treatment (p less than 0.01), but no changes were found in T4 and TSH levels. It was concluded that the beta 2-mimetic-mediated changes in thyroid status provide one more reason for restriction of the use of beta-mimetic drugs in hyperthyroidic patients and might offer an additional explanation for the undesirable chronotropic cardiac side-effects of the therapy.