No, PMDD is not a hormonal imbalance. Levels of hormones are normal in people with PMDD, but they have an abnormal mood reaction to normal hormone changes that occur across the menstrual cycle. 

As part of gaining an accurate diagnosis of PMDD, a full female blood panel should be taken to rule out conditions, such as an imbalance or thyroid disorder, which can mimic the symptoms of PMDD.  You can of course have a hormonal imbalance as well as PMDD, however that is not the root cause of PMDD.

When we use experimental methods to manipulate hormones in those with and without PMDD, we see that only people with prospectively-confirmed mood symptoms show a negative reaction to the hormone manipulation (Schmidt 1998), but that this goes away after a little while (probably about a month; Schmidt 2017).
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Updated 16 November, 2019 by Tory Eisenlohr-Moul, PhD