Text from About.com:
At some stages of Hashimoto's disease, however, and in particular, often during the early stages, the thyroid that is in the process of autoimmune failure may actually spurt into action and become temporarily overactive, making a person hyperthyroid. Sometimes, it's the temporary hyperthyroidism symptoms -- anxiety, heart palpitations, fast pulse, weight loss, diarrhea, insomnia. The hyperthyroidism is just temporary "Hashitoxicosis" -- and the patient is ultimately on his or her way to becoming hypothyroid.
Thyroid Disorder Tracker