Relationship between TSH Level and Effective Half-life of I-131 in Differentiated Thyroid Cancer Patients
Thyroid carcinomas account for approximately 1% of all human cancers with the incidence of Differentiated Thyroid Carcinoma (DTC) varying from 2 – 10 per 100, 000 but having a high prevalence due to good prognosis. It is reported that the survival rates of patients with DTC can reach as high as 90% as a result of effective therapy consisting of total thyroidectomy; with or without radioiodine ablation. Iodine -131therapy is an integral component in the management protocol for DTC worldwide. The effectiveness of radioiodine therapy is linked to sufficiently elevated serum TSH levels in DTC patients at the time of treatment and the effective half-life of 131I in these patients respectively. Sufficiently elevated TSH is important in using thyroglobulin as a tumour marker especially in patients with total thyroidectomy. The effective half life of 131I in DTC patients is linked to the effective radiation dose (absorbed dose) to the remnant thyroid tissue which in turn is linked directly to the effectiveness and efficacy of radioiodine remnant ablation (RRA). This study was aimed at finding the relationship between the two entities. We studied 38 patients with DTC, 18 patients (47.4%) had total thyroidectomy and 20 (52.6%) had non-total thyroidectomy (sub-total, near-total, partial thyroidectomy or lobectomy). There were 29 females (76.3%) and 9 males (23.7%), age range 20 – 58 years (mean 36.74 ± 9.61 years), mean weight 72.29 ± 18.24 Kg. The patients had 131I treatment, mean dose 4.93 ± 1.93GBq (dose range 3.26GBq – 12.21GBq,). The effective half life of 131I in these patients as calculated from measurements of the retained activity in the patients after 24h of 131I administration ranged between 0.31 – 0.96 days, (mean 0.61 ± 0.18 days and median = 0.6 days). The TSH results for the patients ranged between 25.6 – 499.8mIU/ml (mean = 179.49 ± 125.59mIU/ml and median = 148.4mIU/ml). There is a significant positive correlation between the patients’ serum TSH and the effective half life (P = 0.00119, r = 0.506), therefore, it is concluded that there is a direct linear relationship between the serum TSH level of patients with differentiated thyroid cancer and the effective half-life of 131I during radioiodine therapy.
-John E. Ejeh
Department of Nuclear Medicine
University College Hospital
Ibadan South West Nigeria