Abstract Body (Do not enter title and authors here): Introduction: Thyroid disorders are strongly associated with cardiovascular diseases (CVD) due to the wide ranging effects of thyroid hormones on the cardiovascular system. A significant number of annual deaths are attributed to the co-occurrence of these conditions. However, the extent of mortality and varying trends remain understudied.

Research Question: The study aims to explore the trends and disparities in thyroid and CVD related mortality between 1999 and 2023 among U.S adults aged ≥35 years.

Methods: This is a retrospective analysis of the mortality data of U.S adults aged ≥35 years from 1999 to 2023, taken from the Centers for Disease Control and Prevention Wide-Ranging Online Data for Epidemiologic Research (CDC WONDER) database, using ICD-10 codes for CVD and thyroid disease. Age-associated mortality rates (AAMRs) per 100,000 population were calculated and annual percentage changes (APCs) were analyzed via joinpoint regression. Outcomes were stratified by year, sex, race, region and type of thyroid disorder.

Results: From 1999 to 2023, a total of 512,402 thyroid-CVD-related deaths were recorded in U.S adults ≥35 years. The overall AAMR showed a slight variation from 10.9 in 1999 to 11.2 in 2018 (APC: -0.3; 95% CI: -0.5 to -0.08), followed by a significant increase to 14.9 in 2021 (APC:12.5; 95% CI: 4.8 to 20.7) and a drop to 13.6 in 2023 (APC: -5.5; 95% CI: -11.3 to 0.69). Most of the CVD related mortalities were linked to hypothyroidism (AAMR hypothyroidism: 10.4, hyperthyroidism: 0.73, other thyroid disorders: 0.40). Women showed a consistently higher AAMR than men from 1999 (women:13.1, men:6.9) to 2023 (women:16.4, men:9.8). Regional disparities were evident (AAMR Midwest: 12.5, West: 11.9, South: 10.9, Northeast: 10.8). Nonmetropolitan counties had a higher AAMR (12.4) than metropolitan counties (9.6). Non-Hispanic (NH) White population had the highest AAMR (12.2) followed by NH American Indians (11.8), NH Black (8.5), Hispanic/Latino (8.2) and NH Asians/Pacific Islanders (5.24).

Conclusion: Thyroid-CVD-related mortality among U.S adults aged ≥35 years has shown a notable rise after 2018, coinciding with the COVID19 pandemic. Most of the deaths are linked to hypothyroidism with the highest mortality burden among women, White population and ones living in Midwest and non metropolitan regions. These results warrant further research to address the variations and create targeted public health strategies for better outcomes.