Comparison between remnant and red-marrow absorbed dose in thyroid cancer patients submitted to 131I ablative therapy after rh-TSH stimulation versus hypothyroidism induced by L-thyroxine withdrawal

Objective In thyroidectomized patients, increased levels of thyroid stimulating hormone (TSH) are necessary to maximize 1311 uptake. Traditionally, this has been achieved by withdrawing L-thyroxine (L-44) for 4-6 weeks, inducing hypothyroidism in patients. The availability of a genetically engineered version of the recombinant human TSH (rh-TSH) provides an alternative tool to enhance the TSH serum level without inducing hypothyroidism. In this paper the I-131 remnant and red-marrow doses calculated in differentiated thyroid cancer (DTC) patients pre-treated with rh-TSH are compared to those calculated in patients in hypothyroidism induced by L-T4 withdrawal. Methods Forty-six DTC patients, submitted to 131 1 ablative therapy, were randomly divided in group A (pre-treated with rh-TSH) and group B (treated after L-T4 withdrawal for 30 days). The red-marrow absorbed dose per unit administered activity and the remnant cumulated activity per unit administered activity were calculated for both groups. Results The red-marrow dose in 17 rh-TSH treated patients is 0.06 +/- 0.02 mGy(.)MBq(-1); that in 14 hypothyroid patients is 0.09 +/- 0.03 mGy(.)MBq(-1) (two-tailed unpaired t-test P=0.003). The remnant cumulated activity per unit administered activity in 10 rh-TSH treated patients is 0.9 +/- 0.8 h; that calculated in 21 hypothyroid patients is 1.55 +/- 1.05h (two-tailed unpaired t-test P=0.063). This last result is mainly due to the difference between the maximum uptake (U) in rh-TSH (U=0.01 +/- 0.01) and hypothyroid patients (U = 0.03 +/- 0.02) (two-tailed unpaired t-test P = 0.019). Conclusion The rh-TSH pre-treated patients seem to have a lower uptake compared to those in hypothyroidism induced by L-T4 withdrawal. On the other hand their red-marrow absorbed dose seems to be lower.