The Effect of Suppressive Therapy of Nontoxic Diffuse Goiter on Serum Levels of Thyroxine, 3,5,3‘-Triiodothyronine and 3,3’,5‘-Triiodothyronine

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Abstract
For comparison with thyroxine [T4] suppression and in order to elucidate the degree of suppression of the pituitary thyroid axis by T3 [triiodothyronine] the effects of suppressive therapies with T3 and T4 were studied in the same type of patients. Serum T4 rose significantly to a constant level during T4 treatment (0.10, 0.15 and 0.20 mg/day). Dose-related rises in T4 were only seen after 3 mo. of treatment. T3 and r[reverse]T3 only changed minimally. The T4/T3 ratio rose to a constant level during the initial 3 mo. of treatment. T3/rT3 ratio remained unchanged. No dose-related differences in T4/T3 and T3/rT3 ratio were observed. Treatment with T3 in doses of 0.06 mg/day caused a significant but slow fall in T4 and rT3 to hypothyroid levels while T3 only rose slightly. The T4/T3 ratio dropped significantly during T3 therapy.