Moderate Hypothyroidism in Preparation for Whole Body 131I Scintiscans and Thyroglobulin Testing

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Abstract
Patients who have undergone thyroidectomy for thyroid carcinoma are frequently subjected to periods of induced severe hypothyroidism in preparation for 131I whole body scanning and measurement of serum TG. These two tests are crucial in evaluating the patient's clinical status and determining administration of 131I or other necessary treatment. Severe hypothyroidism produces fatigue, weight gain, depression, inability to carry out usual activities, and occasionally significant illness. We compared the efficacy of inducing moderate hypothyroidism by cutting replacement therapy in half, to a standard method. In the standard preparation, patients substituted triiodothyronine for thyroxine replacement over a 3-week period, and then omitted hormone therapy for 3 weeks. For the subsequent scan, 6 to 12 months later, the thyroxine dosage was cut in half. TSH levels were assessed 4 weeks later, and if adequately elevated, whole body scanning was conducted at the end of the fifth week. Pulse, weight, clinical symptoms, thyroid hormone levels, and some clinical chemistries were evaluated prior to each scan, and some of the tests were also carried out during the interval between scans. Moderate hypothyroidism induced by the half-dose protocol induced TSH elevations above the target level (25-30 μU/mL) at 5 weeks in most patients. Typically TSH of 15 μU/mL in the previous week predicted adequate elevation of TSH at the time of scan. Half dose therapy can be prolonged, if necessary, especially in patients who begin with extreme suppression of TSH, or if a higher TSH is desired. Pulse, weight gain, and cholesterol were significantly different in the two protocols, and the patient's subjective evaluation of hypothyroid symptoms was significantly reduced. Reduction of thyroxine replacement dosage to half the usual amount, in patients with thyroid cancer, allows after 5 weeks in most patients sufficient elevation of TSH for whole body scanning and measurement of TG levels. This simple and economical procedure drastically reduces symptomatology of hypothyroidism and makes this key procedure much more tolerable to patients.

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