One of the most perplexing questions about autoimmune thyroid disease addresses this frequency of disorders among women. Patients with autoimmune thyroid disease are five to ten times more likely to be female.
At least one contributing condition is known as fetal microchimerism, which is caused by the passage of cells from a developing fetus into the mother’s body tissues. These cells can persist and even thrive in the mother’s body for decades after she gives birth. The mother’s immune system interprets these cells as invading microorganisms, sometimes triggering an autoimmune response in the process. This process is likely at least partially responsible for autoimmune thyroid disease occurring after pregnancy.
Another possible reason for the female preponderance in autoimmune thyroid disease is called X-chromosome inactivation. Women and men differ genetically in the chromosomes responsible for determining gender (the X and Y chromosomes). While men have one of each (one X and one Y), women simply have two X chromosomes and no Y chromosome. In each cell of a woman’s body, one of the X chromosomes is inactivated; however, it is not the same X chromosome in each cell. In fact, most tissues in the female body contain a relatively equal mix, with approximately half of the cells inactivating one of the X chromosomes and the other half of the cells inactivating the other X chromosome. Women with autoimmune thyroid diseases often have a skewed x-chromosome inactivation in their tissues – that is to say their cells tend to favor the inactivation of one particular X chromosome or the other. It is believed this may contribute to the development of autoimmune disorders in some way.