Check Hormone Levels. Verify the active and storage forms of thyroid are at optimal (not just "normal" if you want best results) levels as well as testosterone and dht. Consider topical Minoxidil (rogaine) to help thicken more hairs more rapidly. Rogaine (minoxidil) will not work well in the presence of hormone deficiencies and low zinc. Stress can turn regular hair shedding into all shedding at once and mimic low thyroid too.
Treat the thyroid. Treat the thyroid to bring it back to the normal range. If the hair loss was caused by the low thyroid, it will recover once the thyroid is back to normal. Keep in mind that the hair cycle is a long process and you might not see the effect of hair regrow until 6-9 months later.
Manage thyroid first. Your first priority is to manage the hypothroidism. Unfortunately once hair goes into the "sleep phase" it may take up to 6 months to "wake up" and grow. Make sure you monitor your thyroid levels and take the medications as prescribed.