There are innumerable questionnaires for tinnitus, and the THI is a popular one. The THI is also called the "THQ" for tinnitus handicap questionnaire.
The tinnitus handicap inventory was developed by the same authors who developed the dizziness handicap inventory, "To develop a self-report tinnitus handicap measure that is brief, easy to administer and interpret, broad in scope, and psychometrically robust." (Newman et al, 1996)
The THI is not a diagnostic test -- rather it produces scores related to the perceived tinnitus handicap -- a subjective quantity.
The THI is a 25 item test, and the maximum score is 100.
Salviati et al (2013) reported that scores above 36 suggest psychiatric distress.
We also have a large repertoire of dizzy patients in our clinic practice with tinnitus, where we have obtained the THI. For these, the distribution is much different than for the DHI (dizziness handicap inventory) . This data is from the beginning of 2018(n=854). These are mainly people who are dizzy, and whose chief complaint is not tinnitus at all. There may be a few who are mainly being seen for tinnitus however. Note that this distribution is not a "normal" distribution. Rather it is skewed towards high numbers. This means it does not have very good statistical properties. This highly skewed distribution is similar to that of the "MIDAS" questionnaire for migraine.