Establishing Population-Specific Thyroid-Stimulating Hormone Reference Intervals for Syrians: An Age- and Sex-Stratified Analysis Using Indirect Method

I- Introduction

In places such as Syria, understanding how thyroid-stimulating hormone varies across populations matters deeply within endocrine science. Because people, cultures, and surroundings differ widely there, these differences shape hormone levels in ways outside data may miss research by Ahmad and team in 2020. Even though labs worldwide report average values for TSH, real-world health settings often demand tailored standards. What works in one region can fail elsewhere due to distinct exposure patterns - like air quality or diet - that influence baseline biology (Elzehery et al., 2024). Diagnosis relies heavily on comparing individual results to appropriate local norms rather than global averages. Without such context-specific benchmarks, doctors might overlook subtle shifts that signal trouble. So building region-specific TSH ranges becomes not just scientific but necessary. Al-Abduljabbar and team in 2024.

Older people, those from certain backgrounds, and exposure to polluted areas often influence how thyroids work - this might shift TSH numbers. Take kids: research by Oron and team in 2020 showed TSH ranges differ across ethnic groups and weight levels, meaning location and identity matter. What stands out matters most in Syria - where people come from many ethnic roots, shaping how they might react when their environment gets disrupted by thyroid-altering pollutants.

Still, how we feel inside ties closely to tiny food elements - science again shows lacking nutrients might shift how thyroids work. Take one look at work by Shaheen and team in 2024: they checked hundreds of Afghan refugee groups, seeing slumps in hormone creation when key nutrients were low. Much like what happens across war-torn Syria, where getting basic sustenance isn’t equal for everyone, so too does body balance differ between communities nearby.

What happens outdoors affects thyroid work in Syria, no exception. In certain areas, levels of iodine shift - this affects how thyroid hormones form. That detail shows up clearly in data from Salameh and team (Close locations between populations might affect thyroid hormones, according to 2022 findings for Lebanese adults. Yet within Syria, those results may not fit so well. Pollutants from surroundings interfering with hormone systems differ by region here too. Because of those differences, studies should look at specific neighborhoods to see exactly how they alter TSH levels.

Healthcare in Syria faces special difficulties when it comes to getting enough proper medical supplies or steady lab services. Take thyroid checks, for instance - Mahmood and team in 2024 showed how vital local norms are by looking at newborns in nearby Iraq. When global standards ignore regional factors, care can shift off track. With weak healthcare systems across Syria, setting unique targets for infant thyroid hormone levels matters deeply. Accurate treatment depends heavily on such tailored benchmarks (Karar et al., 2025).

When it comes to public health challenges like metabolic syndrome or autoimmune problems, thyroid function plays a key part - making this topic even more pressing. Research by Albishara and team in 2022 found strong links between metabolic syndrome and hypothyroidism among Syrian patients, showing how these conditions affect each other plus how care systems are set up Tarboush et al., 2023. Because of these ties, checking TSH levels becomes more than just routine testing - it can shape how metabolic issues are handled across affected areas.

Looking at how TSH levels connect with health results, there’s evidence tying them to cancer risks - Shahrokh and team showed this in 2023 by studying patients before surgery in Syria, focusing on papillary thyroid cases. What stands out is that standard TSH ranges might not fit every population, since regional differences in health could shape both treatment paths and individual outcomes.

Still, autoimmune thyroid disease matters more in certain groups than others. Looking at Hazzaa and team’s 2024 study, people with type 1 diabetes in Syria had higher rates of such conditions. That link might point to shared risks worth noting during creation of TSH benchmarks. When thyroid status interacts with other health issues, seeing how they mix shapes a fuller picture of well-being.

What also matters are psychological and physical habits tied to how people handle stress or manage their routines, like what studies found about cadmium and manganese effects (Błażewicz et al., 2022). Kids and teens from wealthier or poorer homes face distinct risks from pollutants around them, showing that hormone balance and TSH readings depend on more than just genetics - surroundings play a role too.

When it comes to thyroid health in Syria, one size does not fit all. People from different backgrounds experience variations in thyroid function tied to age, genetic background, outside conditions, plus how care systems are set up. If standards are applied too broadly, doctors might miss signs or treat problems incorrectly. That kind of mistake can ripple through entire communities. Recent studies highlight how local research matters, shaped by Syria’s distinct population and natural conditions, which can lead to better health results across the area.

II- Materials and methods

III. Results

Discussion

The central finding of this study is both statistically robust and clinically significant: population-specific TSH reference intervals derived from 9,735 consecutive samples in a Syrian cohort deviate substantially from the manufacturer’s generic range of 0.27–4.20 μIU/mL across virtually all age and sex strata (Varghese et al., 2024). In five of six subgroups, upper reference limits exceeded 4.20 μIU/mL, reaching a maximum of 7.01 μIU/mL in male adults—a deviation of +66.9% above the manufacturer’s upper threshold. The direct clinical implication is serious: any Syrian adult male whose TSH concentration falls within the 4.20–7.01 μIU/mL window would be incorrectly classified as hypothyroid under the universal standard, exposing this population to unnecessary levothyroxine therapy, associated iatrogenic risk, and the cumulative burden on a health system already strained by prolonged socio-economic and humanitarian pressures (Hamad et al., 2020).

The divergence observed here is consistent with a growing body of evidence demonstrating that TSH distributions are population-contingent rather than universal. Naous et al. (2025) recently established reference intervals for Lebanese adults using a decade of laboratory data, finding that even geographically adjacent populations with substantial ethnocultural overlap require their own locally validated ranges. Oron et al. (2020) further demonstrated that paediatric TSH values must account for BMI and ethnicity, a finding directly applicable to the children’s subgroups in the present study, where upper reference limits of 6.06 μIU/mL (females) and 5.85 μIU/mL (males) considerably exceed the manufacturer’s 4.20 μIU/mL ceiling. In Syria specifically, where autoimmune thyroid disease co-occurs with type 1 diabetes at measurable prevalence (Hazzaa et al., 2024) and where papillary thyroid carcinoma workups reveal pre-surgical TSH profiles that diverge from Western norms (Shahrokh et al., 2023), the inadequacy of generic reference intervals is not merely a statistical artefact but a diagnostically consequential reality.

A particularly noteworthy ontogenic pattern emerged across the life course. Within the female cohort, TSH variability was greatest in childhood (mean 2.48 ± 1.79 μIU/mL; CV 72.2%), contracted markedly during adolescence (mean 1.51 ± 0.77 μIU/mL; CV 51.0%; upper RI 3.05 μIU/mL), and then re-expanded in adulthood (mean 2.04 ± 2.15 μIU/mL; CV 105.8%). The adolescent interval, at 3.05 μIU/mL, was the narrowest observed across all six subgroups and fell 27.4% below the manufacturer’s upper limit—a directional reversal compared to all other strata. This finding likely reflects the consolidation of hypothalamic-pituitary-thyroid (HPT) axis set-point regulation during pubertal maturation, a period of maximal gonadal steroid feedback integration ( Sharabi-Nov A et al., 2023). The subsequent widening of the adult interval, particularly in females (upper RI 6.34 μIU/mL), plausibly reflects the compounding influence of menstrual cycle-related thyrotrope modulation, subclinical autoimmune thyroiditis, and age-related HPT axis drift. The broader metabolic context is also relevant: the well-documented association between hypothyroidism and metabolic syndrome in Syrian patients (Albishara et al., 2022) underscores the necessity of age-stratified reference intervals so that TSH elevations reflecting metabolic comorbidity are not misattributed to primary thyroid pathology.

The sex-dimorphic pattern in adult TSH dispersion warrants careful consideration. Male adults in this cohort exhibited both the highest upper RI (7.01 μIU/mL) and the widest coefficient of variation (117.9%) of any subgroup—a finding that runs counter to classical epidemiological data from Western cohorts, where females typically display higher TSH values due to oestrogen-mediated enhancement of thyrotrope sensitivity (Wang et al., 2021). Several non-mutually exclusive explanations may account for this (Duntas et al., 2025). First, the indirect method inherently enriches the female dataset with individuals referred for suspected thyroid pathology, potentially inflating the female distribution’s upper tail relative to a true healthy reference population (Msuega et al., 2021). Second, geographic heterogeneity in iodine sufficiency across Syria may affect males and females differentially in terms of referral patterns and subclinical autoimmune burden. Third, chronic psychosocial stress, a documented modulator of HPT axis function (DeCaro and Helfrecht, 2022), operates through neuroendocrine pathways that may have sex-specific thresholds in a conflict-affected population. The role of environmental exposures—including heavy metals such as cadmium and manganese, which measurably alter thyroid function and are subject to sex-specific pharmacokinetic profiles (Błażewicz et al., 2022)—represents an additional layer of biological complexity that population-specific intervals must accommodate.

The context of conflict-driven nutritional insecurity is particularly salient when interpreting the elevated TSH variability observed in the adult strata. Shaheen et al. (2024, 2025) documented measurable suppression of thyroid hormone synthesis in adolescent Afghan refugees experiencing micronutrient deficiency, a population sharing important sociodemographic parallels with internally displaced and food-insecure Syrians. Iodine insufficiency, which modulates the HPT axis at multiple levels, has been reported in adjacent populations (Salameh et al., 2022) and likely operates with geographic heterogeneity within Syria itself. The contribution of these nutritional variables to the wide intra-population TSH dispersion captured by the indirect method—particularly the high SD values in adult males (SD 2.46 μIU/mL) and adult females (SD 2.15 μIU/mL)—cannot be disaggregated from this retrospective dataset and constitutes a priority for prospective investigation. Reference interval studies employing direct methods with strict exclusion of nutritional confounders would yield considerably narrower and biologically purer intervals than those reported here (Mohammadparast et al., 2024).

The geographic dimensions of thyroid disease epidemiology further contextualise these findings. Kim et al. (2020) documented substantial geographic variability in thyroid cancer incidence, attributable in part to region-specific environmental and diagnostic factors; the diagnostic accuracy of TSH-based screening is necessarily undermined when the reference standard is derived from a non-representative population. In neighbouring countries and regional cohorts, the establishment of local neonatal and paediatric reference ranges has already demonstrated measurable improvements in the early detection of congenital hypothyroidism and neonatal dysthyroidism (Mahmood et al., 2024), outcomes with irreversible developmental sequelae if delayed. The same principle applies to the paediatric subgroups in this study: female and male children presented upper reference limits of 6.06 and 5.85 μIU/mL, respectively—values 44.3% and 39.3% above the manufacturer’s ceiling—reinforcing the case that paediatric thyroid screening in Syria requires locally calibrated thresholds rather than imported adult-derived or manufacturer-specified ranges (Gill et al., 2023).

Several methodological considerations merit acknowledgement. The indirect approach employed here, while pragmatic and well-suited to large retrospective datasets, does not exclude subclinical or undiagnosed thyroid disease from the reference population, which inherently widens the derived intervals relative to those from rigorously screened healthy volunteers (Gandhi et al., 2022). The negative lower boundary values yielded by the Gaussian ±2SD model—floored at 0.00 μIU/mL throughout—reflect the right-skewed nature of TSH distributions and are a recognised limitation of applying parametric statistics to log-normally distributed analytes. Future work employing log-transformation, Box-Cox normalisation, or non-parametric percentile derivation as recommended under CLSI EP28-A3c would likely yield narrower and statistically better-specified lower bounds. Additionally, the marked female predominance in this cohort (F:M ratio 2.22:1, 68.9% female) reflects the well-established sex differential in thyroid disease prevalence and healthcare-seeking behaviour, but introduces the referral bias inherent to indirect methods that must be considered when generalising these intervals to unselected community populations (Rasoulizadeh et al., 2025). Notwithstanding these limitations, the scale of the dataset (N = 9,735), its geographic representativeness for a Damascus-based tertiary laboratory, and the consistent directional findings across all adult strata collectively support the validity and practical utility of the derived reference intervals for clinical application in this population.

V- Conclusion

This study establishes, for the first time in a large Syrian cohort, population-specific TSH reference intervals that differ substantially and systematically from the manufacturer’s generic range of 0.27–4.20 μIU/mL. Across 9,735 age- and sex-stratified samples, five of six subgroups yielded upper reference limits exceeding 4.20 μIU/mL, culminating in an upper boundary of 7.01 μIU/mL in adult males—a 66.9% excess over the manufacturer’s threshold. The sole directional exception, female adolescents (upper RI 3.05 μIU/mL), fell 27.4% below that threshold, identifying this subgroup as uniquely vulnerable to false-negative diagnoses of subclinical hypothyroidism when generic intervals are applied. Taken together, these findings demonstrate that reliance on non-localised reference standards carries measurable diagnostic misclassification risk across all age and sex categories in the Syrian population.

The pronounced ontogenic trajectory uncovered here—characterised by maximal HPT axis variability in childhood, a period of relative stability and narrowed dispersion in adolescence, and re-broadening in adulthood—underscores the necessity of age-stratified, rather than composite, reference standards. The unexpected sex-dimorphic pattern in adults, with male dispersion exceeding female (CV 117.9% vs. 105.8%), diverges from classical Western epidemiological data and likely reflects the interplay of iodine sufficiency heterogeneity, differential autoimmune burden, and chronic psychosocial stress operating within a conflict-affected population. These biological and contextual complexities cannot be captured by any single, population-agnostic interval.

The intervals reported here represent a clinically actionable foundation for improving thyroid diagnostic practice in Syria and should be considered for formal adoption by clinical laboratories serving this population, pending validation in prospective, directly sampled healthy-volunteer cohorts with rigorous exclusion of thyroid disease and nutritional confounders. Future investigations should further stratify by iodine status, BMI, and autoimmune serology to isolate the specific determinants of the observed intra-population TSH dispersion. Beyond their immediate clinical utility, these findings reinforce the broader imperative that diagnostic reference standards in resource-limited and conflict-affected settings must be derived from locally representative data—not imported from populations whose genetic, nutritional, and environmental profiles are fundamentally dissimilar.

Ethics and consent statement

The Research Ethics Committee reviewed and discussed the research proposal ref. 22-7 of the Faculty of Pharmacy, dated 5 September 2023, to conduct the research study entitled: Establishing Population-Specific Thyroid-Stimulating Hormone Reference Intervals for Syrians: An Age- and Sex-Stratified Analysis Using Indirect Method.

During the committee meeting held on 12th. September 2023, all submitted documents were reviewed and approved. After due consideration, the committee has decided to approve the proposed study protocol, methodology, and data collection procedures.

Given the extraordinary circumstances and challenges faced by the Syrian healthcare system during the ongoing crisis, the committee acknowledges the difficulty of conducting conventional informed consent procedures. In line with ethical guidelines for research in emergency and crisis settings, and to facilitate the collection of critical data needed to support a severely strained health system, verbally informed consent from participants has been deemed acceptable. This approach aims to streamline the data collection process while maintaining respect for participant autonomy and confidentiality.

All members of the Research Ethics Committee were present at the meeting held on 12th. September 2023, along with the Vice Dean of the Faculty of Pharmacy. It is hereby confirmed that none of the study team members participated in the decision-making procedures of the committee.

The approved study period extends from September 2023 to September 2025.