The thyroid hormones are essential for regulating the basal metabolic rate of almost every single cell in the body. They are involved in cell protein synthesis, differentiation, maturation, metabolism amongst countless other functions. 

Hypothyroidism is a common endocrine disorder affecting approximately 1.4% to 2% of women and 0.1% to 0.2% of men. Common symptoms of the condition include fatigue, depression, mental slowing, weight gain, cold intolerance, itchy skin, and lifeless hair. Less common symptoms of hypothyroidism include hearing impairment and tinnitus.

There does seem to be a link between hypothyroidism and tinnitus. Although there hasn't been too much formal clinical evidence investigating the link, there are a number of hyperthyroid sufferers at the discussion board http://www.medhelp.org/posts/Thyroid-Disorders/hypothyroidism/ who talk about their experiences with the condition. One user posted that she was a hyperthyrodism (Graves disease) sufferer, and after receiving treatment she had a deficiency of thyroid hormones and noticed a loud ringing in her ears. 

Although there are some hyperthyroid sufferers who experience tinnitus, hypothyroidism appears to be the main thyroid disorder that results in ringing in the ears. It is possible that tinnitus may occur in some patients as a result of increased blood pressure or hypertension.

The treatment for hypothyroidism is levothyroxine which replaces the deficient hormones not produced by the thyroid gland. The tablets must be taken every day for the rest of the sufferers life.