We investigated the prevalence of subclinical hypothyroidism and its association with insulin resistance and other cardiovascular (CV) risk factors in rheumatoid arthritis (RA). We recorded thyroid function tests, insulin resistance markers comprising the Homeostasis Model Assessment for insulin resistance (HOMA-IR), the Quantitative Insulin Sensitivity Check Index (QUICKI) and triglycerides/high-density lipoprotein (HDL) cholesterol ratios, and other CV risk factors in 126 patients with RA. Fifteen (12%) were taking thyroxine for hypothyroidism and 14 (11%) had subclinical hypothyroidism (thyrotropin > 4 mU/L and normal free thyroxine levels). Compared to the 97 euthyroid patients, the QUICKI was lower and the HOMA-IR higher in treated (p = 0.031 for both) and subclinical (p = 0.004 for both) hypothyroid cases while the triglycerides/HDL cholesterol ratios were higher in subclinical (p = 0.039) but not in treated hypothyroid (p = 0.365) cases. Treated hypothyroid patients were more often hypertensive (n = 11 [75%]) than euthyroid patients (n = 36 [37%]) (p = 0.008). No other differences in characteristics were found among the three groups. After controlling for potentially confounding variables, subclinical hypothyroidism remained independently predictive of the HOMA-IR and QUICKI (p <or= 0.06) while treated hypothyroidism did not (p = 0.2). Subclinical hypothyroidism was commonly encountered and associated with insulin resistance and its related dyslipidemia in RA. Evaluation of thyroid function should be considered in future studies aimed at delineating CV risk in RA.