Importance of LThyroxine Suppressive Therapy

Both the function and the growth of some metastatic thyroid tumors are under TSH control. It is a common observation that bone or lung metastases increase in size and take up radioiodine during periods of T4 withdrawal, whereas a reduction in size and lack of uptake are observed during periods of T4 therapy. Serum Tg, a marker of cell function, increases dramatically during hypothyroidism (see Fig. 17-1), whereas Tg levels return to low values during treatment with T4. In the classic article by Mazzaferri,30 thyroid hormone therapy significantly influenced both recurrence rate and survival as an independent variable. In this regard, suppression of endogenous TSH to undetectable levels is to be regarded as a true antineoplastic therapy and should never be omitted in patients with active disease.

The drug of choice is T4, and the effective dosage is between 2.2 and 2.8 pg/kg body weight. Higher dosages are usually required in children. In any patient, attempts should be made to use the lowest dose necessary to suppress TSH secretion. The adequacy of the therapy is monitored by measurement of serum TSH, which should be undetectable with an ultrasensitive assay, and serum free T3, which should be in the normal range to avoid iatrogenic thyrotoxicosis. When these guidelines are followed, T4 suppressive therapy is safe and is devoid of long-term side effects on the heart or bone 43

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