You did not mention whether it is hypo-thyroid or hyper-thyroid.
Anyway, yoga therapy includes asanas, breathing exercises, meditation and a host of other processes like dietary discipline, positive thinking and affirmations.
Simhaasana and sarvangasana meant to benefit and promote a healthy thyroid. However, there are other asanas which also help - viparitkarani, bhujangasana, dhanurasana, halasana, matsyasana and supta-vajarasana. Surya-namaskar also helps.
Anuloma-viloma, ujjayi and kapalbhatti among pranayamas help.
Jalandhara Bandha among bandhas contribute to the therapy.
Apart from yoga, you need to focus on other factors - stress and nutritional deficiences or imbalances, if any.
You should discuss the problem with qualified yoga therapist and endocrinologist.
