Abstract
Objectives: Iodine deficiency (ID) is a common cause of preventable brain damage and mental retardation worldwide, according to the World Health Organisation. It may adversely affect brain maturation processes that potentially result in structural and metabolic brain abnormalities, visible on Magnetic Resonance (MR) techniques. Currently, however, there has been no review of the appearance of these brain changes on MR methods. Methods: A systematic review was conducted using 3 online search databases (Medline, Embase and Web of Knowledge) using multiple combinations of the following search terms: iodine, iodine deficiency, magnetic resonance, MRI, MRS, brain, imaging and iodine deficiency disorders (i.e. hypothyroxinaemia, congenital hypothyroidism, hypothyroidism and cretinism). Results: Up to May 2013, 1673 related papers were found. Of these, 29 studies confirmed their findings directly using MR Imaging and/or MR Spectroscopy. Of them, 28 were in humans and involved 157 subjects, 46 of whom had primary hypothyroidism, 97 had congenital hypothyroidism, 3 had endemic cretinism and 11 had subclinical hypothyroidism. The studies were small, with a mean relevant sample size of 6, median 2, range 1 - 35, while 14 studies were individual case reports. T1-weighted was the most commonly used MRI sequence (20/29 studies) and 1.5 Tesla was the most commonly used magnet strength (6/10 studies that provided this information). Pituitary abnormalities (18/29 studies) and cerebellar atrophy (3/29 studies) were the most prevalent brain abnormalities found. Only fMRI studies (3/29) reported cognition-related abnormalities but the brain changes found were limited to a visual description in all studies. Conclusions: More studies that use MR methods to identify changes on brain volume or other global structural abnormalities and explain the mechanism of ID causing thyroid dysfunction and hence cognitive damage are required. Given the role of MR techniques in cognitive studies, this review provides a starting point for researching the macroscopic structural brain changes caused by ID.
Highlights
Iodine is an important micronutrient and a fundamental substrate for the synthesis of thyroid hormones [1,2]
This review aims to identify what brain structural and metabolic abnormalities related to Iodine deficiency (ID) are documented using Magnetic Resonance Imaging (MRI) and other MR techniques such as Magnetic Resonance Spectroscopy (MRS)
The literature search identified 1673 publications; 553 from Medline, 625 from Web of Knowledge, 490 from Embase and 5 from review paper reference lists. 29 of these studies were included in the review (Table 1 and Figure 1). 1155 papers were rejected because the paper involved non-relevant diseases [Exclusion Criteria], was not related to the effects of ID on the brain and/or used iodine as a contrast medium or therapy, the full paper was not written in English or was not attainable through the search databases
Summary
Iodine is an important micronutrient and a fundamental substrate for the synthesis of thyroid hormones [1,2]. Triiodothyronine (T3) and thyroxine (T4) are examples of iodinated thyroid hormones essential for several cellular metabolic processes and the development of the central nervous system [3]. Thyroid hormone functions are impaired by iodine deficiency [4], reflected as increased plasma thyroid stimulating hormone (TSH) and plasma T3 concentrations with reduced tissue and plasma T4 levels [5]. Iodine deficiency (ID) is one of the three key micronutrient deficiencies highlighted as major public health issues by the World Health Organisation: in 1990, 1.6 billion people, or 28.9% of the global population, were at risk and it was considered a serious public health issue throughout the world [6,7,8].
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