What kind of test I have to do to diagnostic a central hypothyroidism? My TSH and ft3 are normal, but ft4 is low. Is that enought?

Pituitary tests. Central hypothyroidism is very rare, and typically occurs in association with other problems of the pituitary gland. You should have testing of other pituitary function (prolactin, lh, fsh, igf-1, estradiol, cortisol). You may need to see an endocrinologist for a proper diagnosis.

Related Questions

Since hypocretin deficiency reduces TSH production, if a patient also has hypothyroidism, should they be treated as they have central hypothyroidism?

Yes. Hypocretin regulation is not completely understood. However, hypothyroidism is hypothyroidism and should be treated according to normal guidelines, with replacement. An endocrinologist appointment is in order, as your situation is more complex and an endocrinologist will be very current in his/her knowledge.
Check. There aren't studies looking at hashimoto's and hypocretin deficiency. There is lots we just don't know yet. To answer your question, kill a bird with 2 stones: measure both TSH and ft4. Even if low-normal, if ft4 is normal, your thyroid function is probably fine. What was your original tsh-ft4?