Optimal Thyroid Hormone Replacement Dose in Immune Checkpoint Inhibitor-Associated Hypothyroidism Is Distinct from Hashimoto's Thyroiditis.

Immune checkpoint inhibitors (ICI) have revolutionized the treatment of many advanced cancers but are recognized to cause treatment-limiting immune-related adverse events (IrAE). ICI-associated thyroiditis is the most common endocrine IrAE and usually resolves to permanent hypothyroidism. Optimal thyroid hormone replacement in these patients remains unclear. We report the levothyroxine (LT4) dose needed to achieve stable euthyroid state in patients with hypothyroidism from ICI-associated thyroiditis, with comparison to patients with Hashimoto's thyroiditis (HT) and athyreotic state. We conducted a retrospective study of adults with ICI-associated hypothyroidism treated with LT4 at an academic medical center. Patient data were collected from the electronic medical record. Cases had ICI exposure followed first by hyperthyroidism and then subsequent hypothyroidism. Controls were HT (positive thyroid autoantibodies, requiring LT4) and athyreotic (total thyroidectomy or radioiodine ablation, requiring LT4) patients. Patients with central hypothyroidism, thyroid cancer, pregnancy, gastrointestinal stromal tumors, and use of L-triiodothyronine were excluded. Our primary outcome compared LT4 dose needed to achieve euthyroid state (thyrotropin 0.3-4.7 mIU/L over >6 consecutive weeks) for ICI-associated hypothyroidism, HT, and athyreotic patients, considering the impact of age and possible interfering medications by linear regression modeling. Secondary analysis considered the impact of endocrine specialty care on the time to euthyroid state. One hundred three patients with ICI-associated thyroiditis were identified. Sixty-six of the 103 patients achieved euthyroid state; 2 with intrinsic thyroid gland function recovery and 64 on LT4. The mean LT4 dose achieving stable euthyroid state was 1.45 ± standard deviation (SD) 0.47 mcg/[kg·day] in ICI-associated hypothyroidism, 1.25 ± SD 0.49 mcg/[kg·day] in HT, and 1.54 ± SD 0.38 mcg/[kg·day] in athyreotic patients, using actual body weight. The difference in dose between ICI-associated hypothyroidism and HT was statistically significant ( = 0.0093). Dosing differences were not explained by age or use of interfering medications. ICI-associated thyroiditis represents an increasingly recognized cause of hypothyroidism. Our study demonstrates that patients with ICI-associated hypothyroidism have different thyroid hormone dosing requirements than patients with HT. Based on our findings and prior reports, we recommend that in patients with ICI-associated thyroiditis LT4 therapy be started at an initial weight-based dose of 1.45 mcg/[kg·day] once serum free thyroxine levels fall below the reference range.