By a large margin, the most common reason for tinnitus is from hearing loss, although there are many different possible causes of hearing loss. There also are some other ENT conditions that can cause tinnitus, but the evaluation by the ENT specialist would have looked for these other conditions.
Tinnitus also can be related to several neurologic conditions, so a neurologic examination would be apropriate, possibly including an imaging test of the brain, such as an MRI. It is also possible to get tinnitus from conditions that creat noise in the neck near the ears, such as arteriosclerosis affecting the carotid artery.Tinnitus also can be due to arthritis affecting the joint where the jaw connects to the skull, called the temperomandublar joint or TMJ.
Since the tinnitus is associated with stomach and nail systemic, then there also should be some evaluation for systemic conditions. It sounds like you have had an excellent endocrine evaluation. The other issue to consider would be whether there is an autoimmune or systemic inflammatory condition. If not already done, it would be appropriate to first perform some screening tests, such as ANA, RF, and sed rate.
So, it would be appropriate to discuss these possibilities with your primary care doctor, who can perform or arrange the evaluation for these conditions.
If I can provide any clarification, please let me know.