We don’t yet know.
Some studies found that those with PMDD are more likely to have thyroid dysfunction, and others find that the relationship between thyroid hormones and PMDD depends on other variables (such as history of abuse). Researchers are still working to untangle the relationship between PMDD and thyroid hormones, and we don’t fully understand the link yet.
When your doctor is working to diagnose PMDD, they may run blood tests for thyroid function because both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) can cause similar symptoms to PMDD - including fatigue, mood swings, depression, irritability, etc. - however, these would not follow a cyclical pattern as PMDD does.
Careful tracking of your symptoms alongside your menstrual cycle is vital in ensuring you get an accurate diagnosis and treatment. A diagnosis of PMDD should not be given until any existing conditions (such as a thyroid disorder) are confirmed and treated.
Added June 2023.