الفهرس 
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Abstract
Hypothyroidism is a common endocrine disorder. Hypothyroidism may be primary or secondary. Hashimoto thyroiditis (HT) is the commonest cause of hypothyroidism in iodine sufficient countries. Overt hypothyroidism is associated with typical symptoms and signs such as the slowing of motor activity, constipation, cold intolerance, menorrhagia, sleep apnea, dry skin, weight gain, snoring, and a hoarse voice.
The thyroid hormones (THs), L-thyroxine (T4) and L-triiodothyronine (T3), have a profound influence on the development and maturation of the brain, both before and after birth. Congenital hypothyroidism (CH), historically termed cretinism, is the commonest treatable cause of mental retardation. Hypothyroidism causes lethargy, hyporeflexia, poor motor coordination, ataxia, muscle weakness, muscle cramps, nerve entrapment syndromes and polyneuropathy. It also cause Impaired memory, slowed mental processing, depression and cognitive impairment.
Vitamin D (VD) is a fat soluble steroid hormone that has been classically known to regulate bone and mineral homeostasis. However, in addition to its classical role, recent studies have shown nonclassical roles of VD in the pathogenesis of cancer, autoimmune diseases, diabetes, and cardiovascular diseases. Better understanding of the physiological role of the VD system, in particular its potential effects on inflammatory and autoimmune conditions increased the interest in its potential role in prevention and control of Hashimoto thyroiditis. Several lines of evidence support the association between VD and hypothyroidism.
Epidemiological studies suggest a relation between VD status and hypothyroidism especially Hashimoto thyroiditis, Also many studies reported correlation between VD serum level and cognitive functions, and the proposed protective mechanisms of VD were its anti-inflammatory properties, upregulation of neurotrophin factors, reduction of free radicals in brain tissue, neuronal calcium regulation and increasing acetylcholine concentration in the brain.
The aim of the present study was to determine the relationship between VD serum level using 25-hydroxy vitamin D (25-OHD3) and cognitive functions assessed by Montreal cognitive assessment (MoCA) score in hypothyroid patients.
This case-control study was conducted on 90 subjects divided into three groups: group (I) included 30 hypothyroid patients with cognitive impairment, group (II) included 30 hypothyroid patients with normal cognitive functions and group (III) included 30 healthy control subjects of matched age and sex. All the participants were subjected to the following: full history taking, complete physical examination, assessment of cognitive functions by MoCA score, laboratory assessment including: Renal function tests ( serum creatinine, blood Urea and eGFR by MDRD equation) , Serum total calcium, serum phosphorus, Serum 25 hydroxy Vitamin D3 by ELISA,
Serum thyroid-stimulating hormone (TSH), free thyroxine (FT4) by ELISA and Serum anti-thyroid peroxidase (TPOAb) and Thyroid ultrasound.
In the present study, the three study groups were age- and sex-matched and there was significant difference between the hypothyroid patients and control group regarding 25 hydroxy vitamin D3.
The mean value of serum 25-OHD3 was statistically significantly lower in the group of patients with hypothyroidism with cognitive impairment compared to the group of patients of hypothyroidism with normal cognitive functions and control group.
25-OHD3 serum level was significantly positively correlated to total score of MoCA and visuospatial and executive function and verbal memory and significantly negatively correlated with TSH.
Furthermore, linear regression for factors affecting MoCA score revealed that among the different studied factors; 25 hydroxy vitamin D3 and serum phosphorous were independently statistically significantly positively correlated with the MoCA score.