Causes of Hypothyroidism

Causes of HypothyroidismINTRODUCTIONHypothyroidism is the most common prevailing endocrine disorder among all endocrine maladies. It is an neutered metabolic state, when the body produces insufficient amount of thyroid hormone. Hypothyroidism is not only a disorder of endocrine establishment, it besides affects almost all the organ musical arrangement of our body. It shows a wide range of disease severity from asymptomatic state to coma and made it an elusive clinical entity9 . thyroid hormone in rise to power to governing the rate of metabolism of fats, carbohydrates and proteins, it also regulates the timing and pace of the central nervous system development. It is extremely important for the growth of cerebral , cerebellar cortex, axonal proliferation, branching of dendrites, synaptogenesis, and myelination . Thyroid hormone enhances the wakefulness, response to various stimuli similar auditory sense experience, learning and memory capacity1. By enhancing the gene express ion it influences the synthesis of myelin. Myelin synthesis is an important cistron determining the speed of impulse transmission along the complex neural way which mediates the evoked potential3 .Sometimes hypothyroidism is referred to as Silent Disease because the early interpret of disease it is asymptomatic. About 1.6 billion people are at risk of getting thyroid disorders worldwide4. According to NHANES III approximately 4 5% of population in the developed world is suffering from hypothyroidism and about 4 15% of people by subclinical hypothyroidism5 .In a developing country like India, iodine deficiency is the most common cause of hypothyroidism. In India, hypothyroidism is classified under the group of Iodine Deficient Disorder (IDD) 6. Since 1983 India has been following the Universal flavour Iodization Programme. As a result of it, there has been a decline in the prevalence of IDD 7. In 2004, WHO assessment of global iodine status documented that India has best iodi ne nutrition and now India is undergoing transition from iodine deficiency to sufficiency phase8 . But a nationwide comprehensive epidemiological study done in the eight cities of India found that the prevalence of hypothyroidism was 10.95%. One third of them (3.47%) are not even aware of their disease. Subclinical Hypothyroidism was observed in 8.02% of people. Females are more addicted to have hypothyroidism than males (15.86% to 5.02%). Unnikrishnan AG et al has also observed high prevalence of hypothyroidism among the Indian adult population6 .The well-known symptoms and signs of hypothyroidism are dry skin, weight gain, cold intolerance, muscularity cramps, hoarseness of voice and constipation10 . As thyroid hormone is essential for the nervous system maturation , central and peripheral nervous system dysfunctions are also important consequences of hypothyroidism. About 60 90% of the patients may have features of peripheral nervous system dysfunction like weakness, paraesthe sia and postural imbalance . The CNS manifestations of hypothyroidism seen in 60 to 80% of patients which include delayed mentation, sensory deficits, depression11 . It also produces delay in the neuronal conduction velocity, alterations in hearing threshold and sensation of smell . All these neurological complications of hypothyroidism will resolve completely with thyroid hormone replacement especially when diagnosed at the early stage12.The peripheral nervous system dysfunction has been studied in hypothyroid patients by a variety of proficiencys. But the studies in favor of quantification of CNS dysfunction were very sparse. The CNS intimacy in hypothyroid patients can be identified with the help of existing advanced electrophysiological studies. Among the electrophysiological studies evoked potentials provide a more reliable and objective measure of the functional virtue of the related sensory pathway13 . Among the various evoked potentials tests, Brainstem Evoked Response Aud iometry (BERA) has been emerged as an effective method of revealing the involvement of auditory pathway even in asymptomatic stage itself11 .Brainstem Evoked Auditory Response are produced in response to brief auditory stimulation14,15 . In this technique following a brief acoustic stimulus, a series of potentials are generated which corresponds to the sequential activation of peripheral, pontomedullary, pontine and midbrain portion of auditory pathway. So BERA helps to evaluate the law of the auditory pathway13 . Persons who are having brachydactylous brainstem response to auditory stimuli more prone to develop Sensorineural Hearing expiration16 .In 1948 Means states that Hearing impairment is one of the troublesome symptom of hypothyroidism and it may be conductive, sensorineural or mixed hearing loss. About 25 to 30% of the hypothyroid patients having loss, but the exact incidence not yet known. So many researchers did study to find the type of hearing loss in hypothyroid pat ients. Howarth and Lloyd proposed that perceptive deafness is the type of hearing loss in hypothyroidism. (9) Rau et al, Bhatia et al, Vont Hoff and Stuart , Parving et al and Isam et al were also accepted that sensorineural hearing loss was the predominant type of hearing loss in hypothyroidism people. Malik et al said that the site of lesion in auditory pathway remains speculative , may be at several levels viz in the middle ear, at cochlea and retrocochlear sites 17. The impartiality of the auditory pathway is essential for the capture of the acoustic signal by the external ear to the coding of signals in the auditory cortex13 .Hearing loss is an unseeyn abnormality which will lead to so many devastating consequences in interpersonal communication, psychosocial wellbeing, quality of life and economic independence. Hearing loss in infants and children repayable to congenital hypothyroidism results in serious impairment in language, communication skills, cognitive and emotional development. In adults hearing loss will lead to loneliness, social isolation, psychiatrical disturbances, depression, occupational stress and relatively low earnings18. There are literatures pointing out that early treatment of hypothyroidism will reverse the hearing loss 19-22.Electrophysiological testing for conclusion the functional integrity of auditory pathway is rarely performed in hypothyroid patients in neurology and otolaryngology practice. Now a days BAER is mainly used for book binding the preterm infants and also prior to cochlear implantation. But it was proved that the delay in the auditory processing time may provide the information about the subclinical involvement of central as well as peripheral neuropathy in hypothyroid individuals 8. So they can be used to assess both normal and abnormal auditory function in the field of research. So that BERA can be used as a screening test to find the CNS involvement and the hearing loss in hypothyroid patients even in the earlier stage itself.In this study an endeavor has been made to find the changes in the brainstem auditory evoked responses in newly diagnosed hypothyroid individuals.