Thyroid volume does not increase during pregnancy in iodine-replete areas. The decrease in free T4 and free T3 and the increase in free reverse T3 concentrations during pregnancy resemble the changes in thyroid hormones seen in non-thyroidal illness. This could be a physiological adaptation enabling energy conservation during the high metabolic demands of pregnancy.
The effect of injection of 1 mg/kg triiodothyronine on cardiac beta-adrenoceptor state was investigated in hypothyroid rats and compared to the effect in hypothyroid rats pretreated with amiodarone (200 mg/kg/day for 8 days). The Kd values of iodocyanopindolol binding to the beta-receptors were not influenced by either T3 injection or by amiodarone treatment. In the absence of amiodarone, injection of triiodothyronine resulted in a small decrease in receptor density at 6 hr, followed by an increase at 24 hr. Rats treated with amiodarone showed a similar response pattern to hormone injection (i. e. a small decrease in receptor density at 6 hr, followed by an increase at 24 hr), but the amplitude of the response was significantly reduced. Moreover, in vehicle injected rats amiodarone treatment resulted in a decrease in receptor density when rats were mildly hypothyroid, but not when rats were severely hypothyroid. It is concluded that amiodarone interferes (directly or indirectly) with thyroid hormone action in the heart.
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