Most people who suffer from tinnitus also experience hearing loss to some degree. As they often accompany one another, the two conditions may be correlated. In fact, some researchers believe that subjective tinnitus can only occur if the auditory system has been previously damaged (source). The loss of certain sound frequencies due to hearing loss may change how the brain processes sound, causing it to adapt and fill in the gaps with tinnitus. The underlying hearing loss typically results from exposure to loud noises or advanced age:
Vascular issues. Some people have blood vessels near their ears that are capable of causing tinnitus. I have found that if the blood pressure is elevated, this increased pressure can cause that dreaded ringing in your ears or even a whooshing sound. Because pregnant women often have elevated blood pressure, they are easily susceptible to tinnitus. Tinnitus caused by pregnancy should go away with an over the counter tinnitus treatment and once the baby is born. An overactive thyroid has also been shown to causes vascular issues that bring on tinnitus.
A common cause of tinnitus is inner ear hair cell damage. Tiny, delicate hairs in your inner ear move in relation to the pressure of sound waves. This triggers cells to release an electrical signal through a nerve from your ear (auditory nerve) to your brain. Your brain interprets these signals as sound. If the hairs inside your inner ear are bent or broken, they can "leak" random electrical impulses to your brain, causing tinnitus.