I can't gain weight!

Help ladies! So I'm 5'3 and 105.5 lbs. everyone tells me to gain weight or asks if I have an eating disorder. I have a thyroid problem tho, hypothyroidism, the kind that makes you gain weight ironically. I used to be a lot heavier in middle school. 145-150 then I started working out a lot and depression made me not eat. Now I'm almost underweight and no matter how much I eat or strength train to gain muscle I can't. Any ideas?