Thyroid disorders following hematopoietic stem cell transplantation in childhood: the impact of the conditioning regimen on thyroid dysfunction, volume changes and occurrence of nodules

Thyroid late effects are among the most frequent sequelae reported after pediatric hematopoietic stem cell transplantation (HSCT). Although the detrimental effects of radiotherapy on the developing thyroid gland have been extensively assessed, the role of chemotherapy-only conditioning regimens remains controversial. We aimed to describe the occurrence, monitoring, and management of thyroid function disorders (ie, Graves disease, Hashimoto thyroiditis, and nonautoimmune hypothyroidism), nodules, and volumetric changes over a 20-year observation period in a single pediatric transplantation unit. In addition, we assessed the impact of different conditioning regimens on thyroid health. The study population for this retrospective observational analysis comprised 244 pediatric patients who underwent HSCT for malignant or nonmalignant diseases between 1999 and 2018 and for whom at least 4 thyroid function tests and 1 or more thyroid ultrasound(s) assessed sequentially after HSCT were available. The 15-year cumulative incidence of either autoimmune or nonautoimmune thyroid dysfunctions (34%, SE 5.3%) did not differ statistically between total body irradiation (TBI)-based and chemotherapy-based regimens (P = .23). Indeed, the cumulative incidence after busulfan (Bu)-based conditioning was overall superimposable to that recorded after TBI (10-year cumulative incidence, 22.2% versus 25.9%, respectively). Nevertheless, the cumulative incidence of nonautoimmune hypothyroidism was statistically higher after Bu-based conditioning (12.4%, SE 3.7%) than after other chemotherapy-only-based conditioning regimens (3.1%, SE 3.1%; P = .02, 5-year cumulative incidence), treosulfan (Treo) included. The overall cumulative incidence of nodules was low for the first 5 years after HSCT (1.9%, SE .9%) but subsequently increased steeply over time, with a 15-year cumulative incidence as high as 52.1% (SE 7.5%). TBI-conditioned patients had a higher 15-year cumulative incidence of nodules (66.8%, SE 9.1%) compared with patients receiving chemotherapy-only regimens (33.6%, SE 9.5%; P = .02), whereas age >10 years at transplantation showed a protective effect (hazard ratio, .42, 95% confidence interval, .2 to .88). Finally, a systematic sonographic follow-up highlighted a progressive statistically significant reduction in thyroid anteroposterior diameter among patients conditioned with TBI (P = .005), but not in those who received chemotherapy-only regimens. TBI and younger age at HSCT have a statistically significant detrimental effect on the occurrence of thyroid nodules, both benign and malignant. TBI and Bu expose patients to a higher cumulative incidence of thyroid dysfunction compared with other chemotherapy-only regimens, Treo included. Accordingly, Bu can be considered the most thyrotoxic agent among those administered as a part of a chemotherapy-only conditioning regimen. Finally, patients conditioned with TBI, but not those with other regimens, show a progressive decrease in thyroid volume over time, as assessed by sequential ultrasound examinations.