Autoimmune thyroid diseases (AITD), Graves disease (GD) and Hashimotos thyroiditis (HT) are common, but there is limited information on the prevalence of co-existing auto-immune diseases (AID). We prospectively collected 3309 patients with AITD and determined prevalences of other autoimmune diseases in index cases and relatives. Overall, 10.3% of 2806 GD patients and 18.1% of 503 HT subjects reported a co-existing AID. Rheumatoid arthritis was the commonest AID in GD (3.17%) and HT (4.17%, P=NS). Addisons disease (0.29% vs 2.98%, P<0.001), pernicious anemia (1.43% vs 3.98%, P=0.005) and vitiligo (2.58% vs 0.89%, P=0.015) were more common in HT than GD. Type1 diabetes (2.74% vs 0.77%, P=0.01) and coeliac disease (1.07% vs 0.89%, P=0.015) were more prevalent in female than male GD patients. In HT index cases Addisons disease was more common in males (4.35% vs 2.76%, P<0.001), with higher rates for pernicious anaemia (4.61% vs 0%, P=0.005) and vitiligo (2.76% vs 1.45%, P=0.011) in females. All AIDs examined were more common in patients with AITD than UK prevalence rates, especially Addisons disease (RR in GD: 32.3, P<0.001; HT: 331.1, P<0.001), rheumatoid arthritis (5.76, P<0.001; 7.58, P<0.001) pernicious anaemia (11.2, P<0.001; 30.6, P<0.001) and lupus (19.6, P<0.001; 30.6, P<0.001). Mothers of GD and HT subjects were reported to have thyroid dysfunction in 17.4% and 23.5% of cases. Mothers of GD patients were more likely to report hyperthyroidism than those of HT (8.84% vs 4.37%, P<0.001); both mothers and fathers of HT reported hypothyroidism more commonly than hyperthyroidism (19.1% vs 8.59%, P<0.001) and 4.17% vs 1.14%, P<0.001). Parents of index cases had increased RR for all AIDs including thyroid. Prevalences of thyroid dysfunction in siblings revealed markedly high lamda s values for brothers of female (9.61) and male (11.36) GD patients and for brothers of female (11.52) and male (16.72) HT subjects.
Conclusion: AITD patients and their parents are at increased risk of co-existing autoimmune diseases; brothers of AITD subjects are particularly likely to have thyroid dysfunction.