You already had thyroid problems, so I wouldn't say the car accident "brought them on", nor can I imagine that a car accident would suddenly make your thyroid start producing thyroid hormones again, though it's possible that an accident could cause neck trauma that could cause fullness.
How about posting labs with reference ranges, both current and prior to the accident so we can compare what your levels were then and are now. Over active thyroid, when taking thyroid medication is, typically, due to over medication.
Also, it would be helpful if you could tell us how much Armour you're taking and how long you've been on this dosage.
Palpitations can apply to either/both hypo and hyper.
Excerpt from the article: Risks and Symptoms of Graves' Disease and Hyperthyroidism...
"Trauma to the Thyroid -- Thyroid trauma can trigger hyperthyroidism in some people. The types of trauma include vigorous manipulation or palpation of the thyroid; surgery to the thyroid, parathyroids, or the area surrounding the thyroid; injection to the thyroid; biopsy of the thyroid; and neck injury, i.e., whiplash, or from an automobile seat belt after a crash."
Excerpt from the book: The Women's Guide to Thyroid Health: Comprehensive Solutions for All Your Thyroid Symptoms:
"Did you start to notice symptoms of hypothyroidism after a car accident that resulted in whiplash? Because the thyroid is located at the base of your neck just under the Adam's apple, if your head is jolted forward or backwards in an accident or a fall, it can be physically damaged. In fact, British thyroid expert Barry Durrant-Peatfield believes that 30 percent of people who have experienced whiplash will develop hypothyroidism (Durrant-Peatfield 2002). Any type of physical trauma that causes a whiplash effect can damage the thyroid gland."