Radiothyroxine Turnover Studies in Thyroid Disease After Therapy

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Abstract
The disappearance curve of plasma radioactivity after intravenous injection of I131-labeled L-thyroxine was employed to determine the rate of hormone degradation or removal. Thyroxine degradation was slower than normal in untreated myxedema, and faster than normal in thyrotoxicosis. In 19 of 20 cases of thyro-toxicosis treated with radioiodine or surgery, the degradation rate became approximately normal or even below normal. Pronounced changes were observed as rapidly as 1 month after subtotal thyroidectomy. In a single instance, the disappearance curve remained slightly accelerated after attainment of euthyroidism. In 2 cases of post-radioiodine hypothyroidism and 1 post-thyroidectomy patient, the rate was as slow as that seen in spontaneously occurring myxedema. In myxedematous patients on replacement therapy, the disappearance curve might remain unaltered for as long as 1-4 months despite attainment of euthyroid status. After prolonged replacement therapy of myxedema for 6 months or more, the degradation rates became normal or changed in the direction of normal.

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