**3.1 Genetic factors**

Genetic component is considered a major risk factor for development of GD. Twin studies show concordance rate of GD in monozygotic twins in between 0.29 to 0.36, and in dizygotic twins between 0.00 and 0.04 [15]. GD predisposition appears to be polygenic [16]. Recently, bioinformatics and next-generation sequencing (NGS) based pangenomic analyses have identified many predisposing genes which are implicated in autoimmune disease, autoimmune thyroid disease and Graves' Disease [16]. These are various genes which take part into the pathogenesis of GD: cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), TSH-R, Tg, CD40, protein tyrosine phosphatase-22 (PTPN 22), HLA, and CD25 [17]. Association with various HLA region is also seen, DR3 haplotype (i.e. DQB1\*02, DQA1\*0501, DRB1\*03) predisposes to GD, whereas the DR7 haplotype (i.e., DQA1\*0201, DQB1\*0302, DRB1\*07 or DQA1\*0201, DQB1\*02, DRB1\*07) appears to be protective [18].

### **3.2 Sex**

GD is more commonly seen in female, and estrogen receptor ESR2 polymorphisms are frequently seen in GD. Association of disease fluctuation and estrogen level variations which are seen during pregnancy, the menstrual cycle, and menopause further clarify its role in pathophysiology of the disease [19]. Estrogen receptor expression is present on orbital fibroblasts, and glucocorticoids can modulate it [20].

## **3.3 Environmental factors**

Smoking adversely affects immune system and thyroid gland health. Current smoking doubles the risk of Graves' hyperthyroidism and triples the risk of developing Graves' ophthalmopathy (GO) the effect was found to be dose dependent and more pronounced in women [21–23]. Conversely four large studies confirm that smoking decreases the risk of hypothyroidism and autoimmune thyroid diseases (AITD) by decreasing Anti-TPO antibodies [24–27]. Three large studies have shown that smoking lowers the serum TSH level accompanied by slight increase in serum level of FT3 and FT4 and this effect is dose dependent [23, 28, 29].

Pestisides and halogenated organochlorides have thyroid disrupting properties that can alter the thyroid functions by binding to thyroid hormone transport proteins [30].

## **3.4 Stress**

Relationship in between stressful life events and onset of GD was documented in 1825. Major stress is positively associated with increased risk of GD. By modulating the cortisol pathway stress can alter the course of many other autoimmune diseases also [31].

## **3.5 Pregnancy**

Pregnancy is associated with major changes in thyroid anatomy and physiology. Hyperthyroidism of GD is increased in early pregnancy and during postpartum. As pregnancy advances GD tends to improve which may be due to better maternal immune tolerance or altered B cell and T cell functions [32]. Decrease immune tolerance after delivery may cause increase in autoimmune thyroid diseases in postpartum [33].
