J.Jpn. Surg. Soc.. 88(10): 1415-1422, 1987

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Original article

HORMONE THERAPY AS POSTOPERATIVE CARE FOR PATIENTS WITH DIFFERENTIATED THYROID CANCER

TSH suppression therapy has been performed for 150 patients as postoperative care for differentiated thyroid cancer. L-thyroxine (LT₄) was given in a dose of 2.1-4.8μg/kg/day, per os. Serum thyroid hormone determinations were done after confirming that the drug had continually been taken for more than three months.
Serum free T₄ (FT₄) values were elevated higher than the upper normal limits (1.9ng/dl) in 127 of 150 cases (84.7%). In 100 cases of these, serum free T₃ (FT₃), reverse T₃ (rT₃ and TSH were estimated. In 98 cases (98%), serum FT₃ was within the normal limits (3-6pg/ml) and no correlation was found between values of FT₄ and FT₃. These serum rT₃ values, however, were elevated higher than the normal range (13-43ng/dl) in 41 cases (41%) correlating with FT₄ values.
Serum TSH, FT₄ and FT₃ showed no correlation with given LT₄ doses. There was no correlation between the serum TSH and FT₃. However, some correlation (r= -0.4046) was found between serum TSH and rT₃ values.
From these results the following conclusions were obtained : i) There must be some auto-regulating mechanisms to keep the serum FT₃ values at the normal level by promoting a de-iodinating process from T₄ to rT₃ when serum T₄ level increased, since rT₃ has no hormone activity. ii) Administration of synthesized LT₄ is safer and more adequately usable than synthesized L-triiodothyronine or desiccated thyroid, as the TSH suppression therapy, because the above mentioned autoregulating mechanism would take place. iii) There is no reason to assume that the increased serum FT₄ values, in the TSH suppression therapy, would represent hypermetabolism, as long as the serum FT₃ levels stay within the normal range.

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