Thyroid problems maybe?

I am 25 years old. I am a female. I have 3 children ages 3y, 5y, 9m. Anyway, My question is could I have something wrong with my thyroid? I am tired all the time. I feel like I am living in a fog. My hands and feet are cold all the time. Sometimes they even have a blueish tint to them. I have problems gaining weight. I am 5’4 and weigh 91 lbs. I have been to the doctor. All he says is after having 3 children all day you probably are tired. And most women would love to have your figure after having 3 children. I just feel like I am losing my mind. Along with my hair… Any thoughts??? anyone?