A diabetic and hypertensive women is on 100mcg thyroxine whose t-4 is normal but tsh is abnormal.what is the reason behind this ?

Incorrect replacemen: If TSH is incorrect, you're on the wrong dose of replacement. Use only brand name T4 replacement, it's much more reliable than generic.

Need more detail: Need to know the exact lab values more clinical information to give a specific answer. Some of the possibilities include- a. Early checking of labs within 1-2 weeks of replacement ( T4 normalizes before tsh) b. Missing dose of levothyroxine intermittently c.Hypothyroidism because of pituitary problem.