Abstract: The present study was designed to investigate the frequency of thyroid dysfunction in Rheumatoid Arthritis (RA) and Systemic Lupus Erythematosus (SLE) and whether these dysfunctions could be an additional risk factor for the development of cardiovascular diseases. Our study included 132 SLE patients (128 females, 4 males) and 217 RA patients (174 females, 43 males). All patients underwent clinical examination and tests for thyroid function and thyroid autoimmunity (antithyroglobulin or Tg and antiperoxidase or TPO antibodies). One hundred and twenty (90 females, 30 males) from the same geographical area, acted as controls. TPO abs were found in 26 SLE patients (19.7%), 22 RA patients (10.1%) and 7 controls (5.8%) while Tg abs were found in 11 (8.3%) SLE, 13 (6%) RA and 2 controls (1.6%). Abnormal thyroid functions were found in 21 (15.9%) SLE, 18(8.3%) RA patients compared to 5 (4.2%) controls (p<0.05). Of those patients with thyroid dysfunction, 17 SLE, 12 RA and 4 controls were positive for TPO abs. The most common abnormality was clinical hypothyroidism (8.3 SLE, 4.1% RA patients) then subclinical hypothyroidism (5.3 SLE, 1.8% RA patients). Subclinical hyperthyroidism was found in 2 (1.5%) SLE and 4 (1.8%) RA patients. Hypothyroid patients had significantly higher blood pressure, low density lipoprotein, C-reactive protein, lipoprotein (a) anti TPO and anti TG abs than euthyroid patients. In conclusion, SLE and RA patients had higher prevalence of anti-thyroid antibodies and hypothyroidism, compared to our normal population. SLE and RA patients with hypothyroidism are at increased risk for CVD compared to those with normal thyroid function.