Context: Subclinical hypothyroidism (SCH) is a common condition that has been associated with ischemic heart disease (IHD) in some, but not all studies. This may be due to differences in study design and the characteristics of participants.

Objective: To investigate whether age and gender influence IHD prevalence, incidence and mortality in people with SCH.

Data Sources: Computerised (PubMed, EMBASE and Cochrane Library) and manual searches of the literature to May 2007, published in English.

Study Selection: Epidemiologic studies that quantified thyroid status and IHD events in adults.

Data Extraction: Two authors independently reviewed articles and abstracted data. Results were compared across two groups based on the minimum age of participants studied (less than 65 years and 65 years or more).

Data Synthesis: Fifteen studies were included for analysis with 2531 SCH participants and 26491 euthyroid individuals. IHD incidence and prevalence were higher in SCH subjects compared to euthyroid participants from studies including those less than 65 years but not studies of subjects aged more than 65 years [Odds Ratio (95% CI)]: 1.57 (1.19 to 2.06) vs 1.01 (0.87 to 1.18) and 1.68 (1.27 to 2.23) vs 1.02 (0.85 to 1.22), respectively. Cardiovascular/all-cause mortality was also elevated in participants from the less than 65 years studies but not from the studies of older people; OR 1.37 (1.04 to 1.79) vs 0.85 (0.56 to 1.29). Prevalent IHD was higher in SCH participants of both genders, although this was statistically significant only in women.

Conclusion: SCH is associated with increased IHD (both prevalence and incidence) and cardiovascular mortality only in subjects from younger populations. These data suggest that increased vascular risk may only be present in younger individuals with SCH.