My tsh 17.38, ft4 15.72 and ft3 4.25 what does it means?

Underactive Thyroid: The elevated TSH represents an underactive thyroid. I need to see the units and lab normals for your free T3 and free T4 levels. You also need a test called anti-TPO antibodies to rule out hashimotos thyroiditis. You will need thyroid replacement but i prefer the bioidentical hormones like armour which has both T3 and T4 hormone instead of synthroid (thyroxine) which is only t4.

Would be rare: It could make sense if your TSH was elevated due to pituitary or hypothalamic tumor. This is very unlikley. Would repeat labs. You need to include normal range as they vary from lab to lab. Looks like your ft4 and ft3 are both upper limit of normal. Usually would suppress tsh.