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Sir,In the evaluation of compensated hypothyroidism,1due cognisance should be taken of the fact that an identical thyroid function profile can characterise primary hypoadrenalism in the absence of intrinsic thyroid disease, hence its reversibility on correction of cortisol deficiency through the use of replacement therapy.2 The issue is complicated further by the fact that secondary hypoadrenalism can co-exist with a variety of autoimmune disorders, including subclinical primary hypothyroidism.3Finally, in occasional patients with pituitary or hypothalamic disorders, instead of the expected fall in serum thyroid stimulating hormone (TSH), when the thyrotropin-releasing hormone/TSH axis is affected, an increase may occur, even in those who are euthyroid on the basis of thyroid hormone levels, resulting in TSH levels of up to 9 mU/l.4 Failure to recognise coexisting hypoadrenalism in each of these instances, could result in a situation where thyroid replacement therapy precipitates adrenal crisis due to the fact that “the increase in metabolic rate after administration of thyroxine presumably necessitates increased secretion of adrenal steroid, without which the patient develops adrenocortical failure”.5
Sir,The article by Benediktsson and Toft1-1 made interesting reading. However, there are several points which need to be discussed, especially from a neuropsychiatric viewpoint. Although the exact cause-and-effect relationship between depression and thyroid hormones has not been established, the strength of association appears to be quiet high. There is mounting evidence that overt hypothyroidism is associated with a range of neuropsychiatric dysfunctions.1-2 As far as subclinical hypothyroidism is concerned, it is increasingly being linked to lifetime risk of depression with rapid cycling bipolar disorder and other mood disorders.1-3 In fact, in the elderly depressed patient, laboratory assessment of thyroid functions is mandatory to avoid misdiagnosis.1-3 Taking into account the high prevalence of mild depressive disorders in the community and its potential associated risks of suicide, disability, and impairment in work performance, a detailed assessment of these cases, including thyroid function testing, is required.1-4 Every case of depression, especially in the elderly, should therefore be biochemically assessed for subclinical hypothyroidism and considered for replacement therapy.
Another situation in which thyroid estimation has therapeutic relevance is the newborn infant. Prompt detection and management of hypothyroidism in the newborn is essential to prevent permanent cerebral damage and mental retardation, which may occur if timely replacement therapy is not instituted.1-3