Evaluating the association between TSH and FT4 levels and the incidence of metabolic syndrome: insights from the Tehran thyroid study using joint modeling

This study aims to investigate the longitudinal association between thyroid-stimulating hormone (TSH), free thyroxine (FT4), and the incidence of metabolic syndrome (MetS) using joint modeling techniques over a ten-year follow-up in the Tehran Thyroid Study (TTS). Data from 1436 participants aged 20 years or older in the TTS cohort were analyzed. The study spanned four years, beginning in 1999 and concluding in 2012, with follow-up assessments conducted every three years. Longitudinal changes in logarithm-transformed TSH and FT4 levels were examined as time-varying exposures to evaluate their association with the incidence of metabolic syndrome (MetS). Over a median follow-up of 8.92 years, 574 new MetS cases were identified. Higher TSH levels were positively associated with MetS risk (HR: 1.17, 95%CI: 1.03-1.33), while higher FT4 levels were inversely associated with MetS risk (HR: 0.54, 95%CI: 0.29-0.97). Higher TSH levels also correlated with an increased risk of hypertension, while higher FT4 levels were linked to a reduced risk of abdominal obesity and hypertriglyceridemia. Our study highlights the association between thyroid hormone changes and the development of MetS, utilizing joint modeling of longitudinal and time-to-event data over a ten-year follow-up. The observed shift toward hypothyroidism, characterized by increasing TSH and declining FT4 levels, emphasizes the importance of regular thyroid function monitoring as part of MetS prevention and management strategies.