The thyroid gland is the body's main regulator and controls the functions of many hormones. The main thyroid hormone is thyroxine (T4), which governs the rate of metabolism of fats, carbohydrates, and proteins and is essential to the growth and development of body tissues. When the body produces an insufficient amount of T4, it creates a metabolic state known as hypothyroidism. The most extreme form of acquired hypothyroidism results from surgical removal of the thyroid gland, known as a total thyroidectomy. This is the treatment of choice for thyroid cancer.
There is considerable evidence of a critical period in which thyroxine is required for cochlear development. There is a clear association between hearing loss and congenital hypothyroidism, but few studies have explored the role of thyroxine in maintaining hearing beyond the critical period of development in the inner ear structures. Additionally, noise exposure has been shown to create free radicals in the ear, causing oxidative damage.
Previous studies have found that a hypothyroid state increases reactive oxygen species scavengers, which can eliminate harmful free radicals. Conversely, T4 has been shown to diminish cochlear damage from kanamycin. It is unclear whether a hypothyroid state would make an individual more or less susceptible to noise-induced hearing loss (NIHL). This study aimed to examine the effects of acquired hypothyroidism on hearing and to determine if thyroid state has any effect on susceptibility to NIHL.
Ten adult chinchillas underwent total thyroidectomies to create a hypothyroid state and five control animals underwent sham surgery. Hearing thresholds were measured by auditory brainstem response with a click stimulus and tonebursts ranging from 1000 to 10,000 Hz. The animals were exposed to 4 hours of continuous 4000-Hz, octave-band noise at 105 dB SPL on the day of the second hearing test. Hearing thresholds were tested three times: pre-surgery, during the post-surgery hypothyroid state, and 3 weeks after the noise exposure.
No significant changes were found between pre-surgery and hypothyroid state thresholds, suggesting that acquired hypothyroidism does not affect hearing. Additionally, there were no significant differences seen in post-noise thresholds, suggesting that hypothyroidism neither protects against nor induces NIHL.
There was very low power in all of the statistical analyses so I cannot rule out the possibility that the lack of significance was due to the small sample size. It is also possible that thyroid tissue grew back, which would alter the hypothyroid state. Future studies could use anti-thyroid drugs such as propylthiouracil (PTU) to better control the hypothyroid state. Additionally, future studies should include measures to examine cochlear hair cell loss and free radical production. It is important to continue studying this area to determine if persons with acquired hypothyroidism are more susceptible to hearing loss and thus require more precautions in noisy situations.
It's September. Football and fall are creeping in, and it's time for the annual “back to school” edition of Page Ten. For the third consecutive year, we're providing you summaries of outstanding research conducted by audiology student researchers. In many ways, the future of a profession is based on its research, and the next few pages will provide you a glimpse into ours. And, you'll notice that it's not only PhD students who are conducting the research.
Every year a committee of the American Academy of Audiology (AAA) conducts a call for papers, and then selects the top student research papers for presentation at its annual convention. The five winners of this year's AAA student research awards were: Isabelle Marie Côté, PhD, University of Ottawa; J. Kip Kelly, MS, Ohio State University; Kristen Schroeder, AuD, University of Buffalo; Jennifer Brooke Shinn, PhD, University of Connecticut; and Brooke E. Soderstrom, AuD, University of Connecticut. We are pleased that four of these individuals provided us summaries of their research for publication here on Page Ten.
As you'll quickly notice, the “hot” research topic of the year seems to have been the auditory steady state response (ASSR), as this is the focus of three of our articles. Although they all involve the ASSR, these three studies are quite different in scope. Kelly investigated the application of a modified spectral analysis technique, known as multitaper spectral estimation, which he compared with the discrete Fourier transform. A second study by Soderstrom investigated the effects of contralateral broadband masking on the ASSR. The third ASSR study, by Shinn, compared behavioral auditory thresholds with ASSR-estimated thresholds in subjects with confirmed CANS lesions. On another topic altogether, Schroeder's paper is an animal study that examines the effects of hypothyroidism on susceptibility to hearing loss.
It's been said that research is simply structured curiosity. If so, we're thankful that today's audiology students are curious, and that they are willing to share with us some of the answers to their questions. We wish them success in their future research activities.