My tsh is high with 18.7 and t3 (liothyronine) is 112 , t4 is 5.3 which shows in normal range ...what should i do...is it harmful to my my body if i not treated?

Underactive thyroid: Total t3 (liothyronine) normal values: 80-200 ng/dl ( for your age ) total T4 normal values: 4.25-12.5 mcg/dl ( for your age ). Your values suggest subclinical hypothyroidism. Nevertheless, T4 is in the low end of normal and t3 (liothyronine) too. I would like to see the values of free T4 and free t3 (liothyronine), a i-123 thyroid scan and a 24 hours urine collection for iodine. Yes, you should be treated.