Is Aswgandha useful in thyroid disorders? 
Dr JV Hebbar
Ashwagandha is useful to stimulate thyroid hormone secretion and utilization. Ashwagandha relieves stress, it is a very good anti-oxidant, stimulates thyroid – Decreases TSH and improves T3 and T4. - Study
Hypothyroidism is often associated with fluid retention and increased sensitivity to cold weather. Ashwagandha being hot in potency, relieves cold sensitivity and being diuretic, relieves fluid retention. 
Hypothyroid people often struggle with depression, stress and weight gain. Ashwagandha with honey, is very useful to address all these three issues. 
Ashwgandha corrects immunity and relieves inflammation, hence useful in auto-immune thyroiditis. 
Ashwagandha is often made into a combination with
Varanadi kashayam,
Kanchanara guggulu,
Punarnavadi mandooram etc. to address hypothyroidism. 

With regards to hyperthyroidism, where thyroid function is excessively stimulated, it is not directly indicated in the treatment of hyperthyroidism.
But it is useful in treating below symptoms which are associated with hyperthyroidism.
Nervousness, irritability
Fatigue, muscle weakness
more frequent bowel movements
Difficulty in falling asleep.
Read more: 
Ashwagandha benefits, side effects, research and remedies

Wish you good health :) 
Dr JV Hebbar