This study evaluated the prevalence of thyroid dysfunction among adult Jordanians at the national level. The overall prevalence of thyroid dysfunction in this population was 11.9%. The prevalence of primary hypothyroidism and hyperthyroidism was 3.1 and 1.0%, respectively. The prevalence of subclinical thyroid dysfunction was 7.8%.
In the current study, a trend toward age-related increase in the median TSH level was not observed. However, the trend toward increasing 97.5 centiles with age irrespective of the sex supports the population shift toward higher TSH concentrations with aging observed in the NHANES III study . It has been hypothesized that the shift may be part of a healthy aging process due to altered sensitivity of TSH receptors or hypothalamic pituitary feedback system, or change in the biological activity of TSH molecule [15, 16]. It is noteworthy that the population of Jordan is relatively young as only 3.7% are older than 65 years . Nevertheless, a large-scale study that includes a larger proportion of elderly is needed to set a reference range for TSH level.
The prevalence of elevated TSH (> 4.78 mIU/l) in our study was 6.1%, while, the prevalence of elevated TSH in Netherlands, Japan, The Health Study of Nord-Trondelag (HUNT), Australia, Colorado, Rotterdam and the Framingham studies was 4.4, 4.5, 5.3, 5.6, 9.5, 11, and 13%, respectively [17,18,19,20,21,22,23]. The prevalence of suppressed TSH (< 0.55 mIU/l) in the current study was 3.3%, while, the prevalence of suppressed TSH in Netherlands, Japan, The Health Study of Nord-Trondelag (HUNT), Australia and Colorado study was 1.2, 0.9, 0.6,0.64 and 2.2%, respectively [17,18,19,20,21]. This discrepancy may be related to the differences in the study population, definition of the disease state, iodine intake and laboratory assay differences.
In this study the overall prevalence of hypothyroidism was 3.1%. Our finding is comparable to that of a recent meta-analysis which included 14 studies from Europe, in which the prevalence of hypothyroidism was 3.05% . This finding is inconsistent with studies from areas with sufficient iodine intake. The prevalence of hypothyroidism in areas of sufficient iodine intake as in Tehran thyroid study, Colorado study, Spain and NHANES III study was 2, 0.4 0.3 and 0.3% respectively [20, 24,25,26]..On the other hand, the prevalence of hyperthyroidism among this study cohort is comparable to that reported from Spain which was 0.8% . But, it is lower than that previously reported from several studies. For instance, the prevalence of hyperthyroidism in Tehran thyroid study, Europe, NHANES III and Colorado study was 2.2, 0.75, 0.5 and 0.1% respectively [21, 24, 25, 27]. A previously reported study from Jordan had shown a significantly higher prevalence of hypothyroidism when compared to the international studies that reached 9.4% and a lower prevalence of hyperthyroidism which reached 0.58% . The inconsistency in reported prevalence of different types of thyroid dysfunction could be related to the differences in study design, racial and genetic background, age, environmental factors, iodine consumption, criteria used to define different types of thyroid dysfunction and the laboratory assay sensitivities used to measure thyroid hormones levels .
The majority of thyroid dysfunction in the current study were undiagnosed and 86% of them were subclinical. Similarly, subclinical thyroid disorders represented 80, 85, 88 and 90% of the undiagnosed thyroid dysfunction in Europe, Spain, NHANES III and Colorado study, respectively [21, 24, 26, 27]. In the same context, the past two decades have witnessed a rise in the prevalence of subclinical thyroid dysfunction. This rise is related to the substantial increase in thyroid function testing with ultra-sensitive assays and the decline in TSH thresholds for levothyroxine initiation .
Sex distribution for thyroid dysfunction in this study clearly demonstrated a female predominance, which was mainly driven by hypothyroidism and subclinical hypothyroidism. The females’ higher prevalence of detected thyroid autoantibodies, in the background of sufficient iodine intake in Jordan could account for this female predilection [27, 30].
In iodine replete-populations, the most common cause of thyroid dysfunction is thyroid autoimmunity. The presence of thyroid autoantibodies particularly TPOAb are considered as a surrogate marker for future development of thyroid dysfunction . On the other hand, thyroid autoantibodies could be present in normal population, but the likelihood is significantly higher in subjects with thyroid dysfunction . Among our study participants, the prevalence of detectable TPOAb and TgAb in the euthyroid participants was10.3 and 11.9%, respectively. Similarly, in NHANES III study, 12 and 10% of the healthy population had detectable TPOAb and TgAb .
This study has many strengths. This study is one of the few studies in the region that assessed the prevalence of thyroid dysfunction and their correlation with thyroid antibodies in the general population using a large representative sample. It provides a better understanding for the distribution of thyroid disease patterns in Jordan and sheds light on the prevalence of unknown thyroid dysfunction. The use of highly sensitive laboratory assays further enhances the validity of the current study results. Finally, determination of a Jordanian specific reference range for TSH level will aid in the future classification and management of thyroid dysfunction.
However, this study has several limitations. First, the female response rate was significantly higher than males. This has been consistently observed in a series of surveys previously conducted in Jordan. The main reason for this finding is the higher employment rate in males which makes their participation more difficult. Second, urinary iodine was not measured, which is needed to assess iodine intake. Finally, the study is cross-sectional in nature and, thus, observed associations can’t be considered as causal because of the possibility of survival and temporal biases associated with this study design.