Context: This review summarizes key findings of the Tehran thyroid study (TTS), a large scale community-based study with approximately a two decade follow-up, about the incidence, prevalence, and natural course of thyroid disorders as well as associations between thyroid diseases and metabolic syndrome (MetS), dysglycemia, and cardiovascular disease (CVD).
Evidence acquisition: PubMed, Scopus, and Web of Science databases, and the library of Research Institute for Endocrine Sciences were used to search for TTS articles. Articles were subdivided based on the fields of prevalence, incidence and natural course, and associations of thyroid function with the incident hypertension (HTN), MetS and CVDs.
Results: The 2.5th and 97.5th percentiles of serum thyrotropin (TSH) were 0.32 and 5.06 mU/L, respectively. Estimated reference intervals (2.5th and 97.5th percentiles) for thyroid peroxidase antibody (TPOAb) levels were 1.5 - 32.8 and 2.1 - 35 IU/mL in men and women, respectively. Euthyroid persistency was 93.24% during 6 years. There was a negative association between free thyroxine (FT4) levels and insulin resistance. Decreasing FT4 values over time would predict MetS in euthyroid and subclinical hypothyroid subjects (TSH < 10 mU/L). The incidence of thyroid disorders in patients with diabetes, pre-diabetes and healthy controls was 14, 18, and 21 per 1000 person-years, respectively, indicating significantly lower incidence in individuals with diabetes compared to healthy controls. Serum FT4 within the reference range was positively associated with all blood pressure (BP) measures in the total population and in men; however, serum TSH was positively associated with only systolic BP (SBP), diastolic BP (DBP) and mean arterial pressure of men. No associations were found between various states of thyroid function and prevalence and incidence of CVD.
Conclusions: A well designed cohort study aimed to investigate the gap in knowledge regarding thyroid disorders can generate many hypotheses to be examined in randomized controlled trials.
Keywords: Cardiovascular Disease; Metabolic Syndrome; Tehran Thyroid Study.