Endocrinology · Medical Literature
Prevalence of thyroid disorders in inland and coastal populations: A systematic review.
Abstract
Endocrine conditions such as thyroid disorders can significantly impact maternal and child health. Key determinants such as environmental setting and presence of iodine can influence the incidence of thyroid disorders. Individuals living in coastal regions are exposed to iodine rich foods in comparison to those residing in inland areas, which is a possible differentiating factor in the occurrence of thyroid based diseases in these individuals. Despite this possible relationship, very few studies have investigated the burden of thyroid diseases amongst coastal and inland populations. The study aims to systematically explore the prevalence of thyroid disorders in coastal and inland regions to understand the impact of differences in diet, environmental determinants, demographic variation and access to healthcare services on thyroid disease occurrence. Databases such as PubMed, Google Scholar and Scopus were used to identify articles in accordance with the PRISMA 2020 guidelines. Twenty three articles were selected based on them being in the English language, full length studies published between January 2017 and September 2023 and had reported on prevalence of thyroid diseases in inland and/or coastal areas. The review identified that a higher prevalence of thyroid cancer and thyroid nodules were seen amongst coastal populations, while goiter and hypothyroidism were seen in inland and mountainous areas. These differences were influenced by the iodine status of the region, consumption of seafood, presence of environmental pollutants like bisphenol A and polychlorinated biphenyls, iodization of salt, socioeconomic conditions and healthcare access. Female gender and older age was found to be significantly associated with greater burden of disease in both areas of study. The geographical location or setting is a vital determinant of the distribution of thyroid disorders, mainly through the influence of iodine intake and exposure to environmental pollutants. Regular monitoring of environmental pollutants, early detection of cases, universal iodization of salt and proactive supplementation of iodine may lead to decreased prevalence of thyroid disorders and its consequences. Longitudinal studies in this field can help identify the causal pathways emerging from socioeconomic factors and environmental determinants to thyroid disorders.
Clinical relevance
Do coastal and inland geographic settings differ in the prevalence and type of thyroid disorders, and what dietary, environmental, demographic, and healthcare-access factors drive any observed differences? __SNAPSHOT__ {"studyDesign":"Systematic review (no meta-analysis); conducted per PRISMA 2020 guidelines","setting":"International; multi-country studies drawn from PubMed, Google Scholar, and Scopus; specific countries not individually enumerated in the available abstract","sampleSize":"23 included studies; aggregate participant count not reported in the abstract","population":"Inland and coastal-dwelling adult populations; maternal and child health contexts noted as particular areas of concern; studies published January 2017–September 2023 and written in English","intervention":"Geographic residence (coastal vs. inland/mountainous), with associated differences in iodine intake, seafood consumption, environmental pollutant exposure, salt iodization status, socioeconomic conditions, and healthcare access","comparator":"Inland and/or mountainous populations versus coastal populations","primaryOutcome":"Prevalence of thyroid disorders (both benign: hypothyroidism, hyperthyroidism, goiter, thyroid nodules, thyroid autoimmunity; and malignant: thyroid cancer) across geographic settings","followUp":"Not applicable (cross-sectional and observational studies reviewed; no longitudinal follow-up reported)","fundingSource":"Not reported"}
Key findings
The review identified a geographically distinct pattern of thyroid disease distribution: higher prevalence of thyroid cancer and thyroid nodules in coastal populations, while goiter and hypothyroidism predominated in inland and mountainous areas. These differences were attributed to iodine status of the region, seafood consumption, presence of environmental pollutants (including bisphenol A and polychlorinated biphenyls), degree of salt iodization, socioeconomic conditions, and access to healthcare services. A U-shaped relationship between iodine and thyroid disease was noted: both iodine deficiency (linked to goiter and higher-risk thyroid cancer subtypes in inland regions) and excess iodine intake (associated with thyroid nodules and papillary carcinoma in some coastal areas) were associated with harm. Female sex and older age were consistently associated with greater disease burden across both geographic settings. Iodine status was primarily assessed via urinary iodine concentration (UIC), using WHO thresholds: insufficient (<100 µg/L), normal (100–199 µg/L), above requirements (200–299 µg/L), and excess (≥300 µg/L). No pooled quantitative effect sizes, confidence intervals, or p-values are reported in the available abstract, and no meta-analysis was performed.
Practice implications
This systematic review establishes that geographic location is an important determinant of the type—though not necessarily total burden—of thyroid disease, primarily operating through iodine availability and environmental pollutant exposure. The finding that inland/mountainous populations face higher rates of iodine-deficiency-related goiter and hypothyroidism aligns with longstanding epidemiological evidence and the physiological mechanism linking iodine deficiency to thyroid stimulation. The observation that coastal populations show higher rates of thyroid nodules and cancer is biologically plausible given excess iodine and pollutant co-exposures but is supported mainly by cross-sectional and registry data, limiting causal inference. Pregnant women and children—populations most vulnerable to iodine perturbations—are mentioned in the rationale but the degree to which included studies specifically characterize these subgroups is not clarified in the abstract. Practice implications include heightened clinical vigilance for hypothyroidism and goiter in patients from inland or mountainous regions, and for nodular disease in patients with sustained high seafood or iodine-excess diets from coastal environments. Because only the abstract was available for this summary, full methodological details, individual study-level data, and quality appraisal results could not be reviewed; conclusions should be interpreted with this limitation in mind. __LIMITATIONS__ No meta-analysis performed; findings are narrative and qualitative without pooled effect estimates, confidence intervals, or statistical testing across studies.,Restricted to English-language full-text articles from January 2017–September 2023, introducing language and publication bias and excluding potentially relevant studies outside this window.,Heterogeneity in how thyroid disorders were defined, diagnosed, and reported across the 23 source studies (e.g., TSH cutoffs for hypothyroidism, ultrasound vs. palpation for goiter/nodules, histopathologic confirmation for cancer) limits cross-study comparability.,All included studies are observational (primarily cross-sectional), precluding causal inference about geographic exposure and thyroid disease.,The 'U-shaped' iodine-thyroid cancer relationship is based on a small number of cross-sectional or registry studies and does not constitute robust causal evidence.,Socioeconomic confounders, healthcare-access disparities, and environmental pollutant exposures were noted as influential but likely incompletely measured or adjusted for across primary studies.,Geographic classification as 'inland' vs. 'coastal' may not be standardized across studies, and mountainous populations are grouped with inland populations despite potentially distinct iodine dynamics.,Funding source is not reported, precluding assessment of potential industry influence.,Generalizability is limited to populations and settings represented in English-language literature from 2017–2023. __BOTTOM_LINE_CATEGORY__ potentially_practice_informing __BOTTOM_LINE__ This systematic review of 23 observational studies demonstrates a consistent geographic divergence in thyroid disease phenotype: goiter and hypothyroidism predominate in iodine-deficient inland and mountainous populations, while thyroid nodules and cancer are more prevalent in coastal populations with higher iodine and environmental pollutant exposure. Female sex and advanced age amplify risk across both settings. Clinicians should tailor thyroid screening and iodine counseling to the patient's geographic and dietary background, and confirmation via well-designed prospective or comparative studies with standardized case definitions is needed before these findings can formally guide population-level screening protocols.