I believe low blood pressure is a symptom of hypothyroidism, and
the dizziness is caused by this. I would also get dizzy
in the shower, and a few times actually fainted, though those times were almost always when I was sick. The doctor told me that what happens is that when you take a hot shower, your blood vessels expand, which lowers your blood pressure, so if you are already having low blood pressure issues, this can cause it to drop so low that you can pass out.