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Serum thyrotropin concentrations rise soon after thyroxine therapy is stopped in patients with thyroid cancer

(March 2003)

The background of the study. Patients with thyroid cancer are often treated with surgery and then iodine-131 to destroy the remaining normal thyroid tissue or persistent cancer. The iodine-131 is given only when the patients' serum thyrotropin (TSH) concentrations are >25 or 30 mU/L. This study determined the time interval between the cessation of thyroxine (T4) therapy and the attainment of serum TSH concentrations ≥30 mU/L in patients with thyroid cancer.

How the study was done. The study subjects were 13 patients with thyroid cancer who had undergone near-total thyroidectomy. All were receiving long-term T4 therapy; their initial serum TSH concentrations were low. Serum TSH, free T4, and free triiodothyronine (T3) were measured when T4 therapy was stopped and then every three to four days until the serum TSH concentration was at least 30 mU/L, at which time iodine-131 was given.

The results of the study. The mean interval from stopping T4 therapy to when a patient's serum TSH concentration was at least 30 mU/L was 17 days (range, 11 to 28). The interval from stopping T4 therapy to when the serum TSH concentration was above the upper limit of normal (4.5 mU/L) was 12 days (range, 9 to 21). Once normal, serum TSH concentrations doubled every 2 to 3 days. The mean time of disappearance of serum free T4was 11 days (range, 8 to 13) and that for serum free T3 was 16 days (range, 10 to 21).

The conclusions of the study. Most patients with thyroid cancer receiving T4 have high serum TSH concentrations less than three weeks after the cessation of T4 therapy.

The original article. Liel Y. Preparation for radioactive iodine administration in differentiated thyroid cancer patients. Clin Endocrinol 2002;57:523-7.