Hypothyroidism During Tyrosine Kinase Inhibitor Therapy Is Associated with Longer Survival in Patients with Advanced Nonthyroidal Cancers.

BACKGROUND: Tyrosine kinase inhibitor (TKI)-induced thyroid dysfunction is recognized as a common adverse effect of treatment, but the importance of incident hypothyroidism during TKI therapy remains unclear. This study analyzed the prognostic significance of hypothyroidism during TKI therapy in cancer patients. METHODS: This was a retrospective cohort study of adult patients with advanced nonthyroidal cancer treated with TKI and available thyroid function testing at three affiliated academic hospitals from 2000 to 2017. Patients with preexisting thyroid disease were excluded. Demographic, clinical, and cancer treatment data were collected. Thyroid status with TKI treatment was determined from thyroid function testing and initiation of thyroid medication, and classified as euthyroid (thyrotropin [TSH] normal), subclinical hypothyroidism (SCH; TSH 5-10 mIU/L, or higher TSH if free thyroxine normal), or overt hypothyroidism (OH; TSH >10 mIU/L, low free thyroxine, or requiring replacement). Multivariate models were used to evaluate the effect of TKI-related hypothyroidism on overall survival (OS). RESULTS: Of 1120 initial patients, 538 remained after exclusion criteria. SCH occurred in 72 (13%) and OH in 144 (27%) patients with TKI therapy. Patients with hypothyroidism had significantly longer OS, with median OS in euthyroid patients of 685 days [confidence interval (CI) 523-851] compared to 1005 days [CI 634-1528] in SCH and 1643 days [CI 1215-1991] in OH patients (p < 0.0001). After adjustment for age, sex, race/ethnicity, cancer type, cancer stage, ECOG performance status, and checkpoint inhibitor therapy, OH remained significantly associated with OS (hazard ratio = 0.561; p < 0.0001), whereas SCH did not (hazard ratio = 0.796; p = 0.165). Analysis of hypothyroid patients (SCH and OH) with TSH >5 and <10 mIU/L stratified by hormone replacement status showed improved survival associated with hormone replacement. CONCLUSIONS: New hypothyroidism in cancer patients treated with TKI is associated with significantly improved OS, should not necessitate TKI dose reduction or discontinuation, and may provide independent prognostic information.