Abstract
Changes in serum free-thyroid hormones after administration of thyroid hormone preparations were studied in 48 patients with primary hypothyroidism. When l-thyroxine (l-T4) was given orally or intravenously, a slight increase was observed in the concentration of serum FT4, but no significant change was found in the concentration of serum T3, or FT3, unless the dose of l-T4 exceeded 150 μg. The concentrations of serum T3 and FT3 were quickly increased to super normal levels by even a relatively small dose of l-triiodothyronine (l-T3) . When more than 50 mg of dessicated thyroid was given orally, the concentrations of serum T3 and FT3 exceeded the normal upper limit. These result indicate that the best and safest medicament among the thyroid hormone preparations for the treatment of hypothyroidism is l-thyroxine, either oral or intravenous.
