Background: Antithyroid drug-induced agranulocytosis (AIA) (neutrophils <500/µL) is a rare but serious complication in the treatment of hyperthyroidism. Methodology: Adult patients with AIA who were followed up at 12 hospitals in Spain were retrospectively studied. A total of 29 patients were studied. The etiology of hyperthyroidism was distributed as follows: Graves’ disease (n = 21), amiodarone-induced thyrotoxicosis (n = 7), and hyperfunctioning multinodular goiter (n = 1). Twenty-one patients were treated with methimazole, as well as six patients with carbimazole and two patients with propylthiouracil. Results: The median (IQR) time to development of agranulocytosis was 6.0 (4.0–11.5) weeks. The most common presenting sign was fever accompanied by odynophagia. All of the patients required admission, reverse isolation, and broad-spectrum antibiotics; moreover, G-CSF was administered to 26 patients (89.7%). Twenty-one patients received definitive treatment, thirteen patients received surgery, nine patients received radioiodine, and one of the patients required both treatments. Spontaneous normalization of thyroid hormone values occurred in six patients (four patients with amiodarone-induced thyrotoxicosis and two patients with Graves’ disease), and two patients died of septic shock secondary to AIA. Conclusions: AIA is a potentially lethal complication that usually appears around 6 weeks after the initiation of antithyroid therapy. Multiple drugs are required to control hyperthyroidism before definitive treatment; additionally, in a significant percentage of patients (mainly in those treated with amiodarone), hyperthyroidism resolved spontaneously.

Background/Objectives: Type 1 diabetes (T1D) is a heterogeneous disease in terms of clinical presentation, treatment requirements, and risk of complications. The identification of biological endotypes and clinical phenotypes has been proposed to support precision medicine approaches. We aimed to assess the prevalence and clinical characteristics of age-at-diagnosis-based endotypes, adult-onset phenotypes, and insulin-resistant phenotypes in a real-world cohort of adults with T1D. Methods: We conducted a single-center, observational, cross-sectional study including adults (≥18 years) with clinically confirmed T1D under active follow-up. Clinical, metabolic, and treatment-related variables were analyzed across predefined age-at-diagnosis-based endotypes and clinical phenotypes. Results: A total of 868 patients were included (median age 49 years; diabetes duration 23 years; age at diagnosis 20 years; 51.5% women). Continuous subcutaneous insulin infusion (CSII) was used by 20.4% of patients, and continuous glucose monitoring (CGM) was used by 95.3%. Mean HbA1c was 7.47%, with a median time in range (TIR) of 63%. The prevalence of age-at-diagnosis-based endotype 1 (ED1) was 11.8%, adult-onset phenotype was 31.3%, and insulin-resistant phenotype was 7.3%. No major differences in glycemic control were observed across age-at-diagnosis-based endotypes. Associations between endotypes and treatment-related variables were largely explained by current age and diabetes duration. In contrast, the adult-onset phenotype was independently associated with lower TIR, higher time above range, lower use of CSII, and greater use of adjunctive therapies. The insulin-resistant phenotype was associated with higher HbA1c, lower TIR, and greater therapeutic complexity. Conclusions: Adult T1D shows marked heterogeneity. In this real-world cohort, age-at-diagnosis-based endotypes were not independently associated with major clinical differences after adjustment for current age and diabetes duration. In contrast, adult-onset and insulin-resistant phenotypes identified subgroups with poorer glycemic control and greater therapeutic complexity.