Transcriptomic profiling reveals thyroid hormone-mediated and compound-specific effects of methimazole and amitrole on testis development.

Thyroid hormones (THs) influence testis development, with early life hypothyroidism resulting in smaller testes. Developmental exposure to thyroperoxidase (TPO)-inhibiting drugs such as propylthiouracil (PTU) and methimazole (MMI) also impair testis development in rodents by reducing TH levels, leading to smaller testes in pups due to, for instance, disrupted Sertoli cell proliferation and maturation. Comparable effects are seen following exposure to the TPO-inhibiting pesticide amitrole, one of many environmental chemicals with TH-disrupting properties. Despite this phenotype, the molecular underpinnings of hypothyroid-induced testis effects are less clear, complicating mechanism-based chemical toxicity testing relying on alternative test methods and omics approaches. Here, we report on transcriptomics profiling of testes from hypothyroid rats induced by chemical exposures. Pregnant Sprague-Dawley rat dams were exposed by oral gavage to 2 doses of MMI (8 or 16 mg/kg body weight/day) or amitrole (25 or 50 mg/kg bw/day) from gestational day (GD) 7 to pup day (PD) 16, with BRB-seq performed for both life stages, specifically GD21 and PD16. Both MMI and amitrole caused significant changes to the testis transcriptome, seen particularly at PD16, with 313 differentially expressed genes (DEGs) defining a shared TH-mediated profile. Additionally, amitrole exposure resulted in a distinct profile of 1,517 DEGs, suggesting compound-specific effects beyond TH disruption. This study underscores the potential sensitivity of transcriptomic profiling in detecting early tissue disruption under toxicological conditions, in this case, testis disruption under hypothyroid state, offering critical insights for chemical risk assessment beyond histopathological endpoints.