The Impact of Shift-Work and Night Shift-Work on Thyroid: A Systematic Review

Investigating the relationship between shift work schedule and blood and metabolic parameters: a 10-years retrospective cohort study

Shift work has become increasingly common in modern society. Shift work has been associated with a range of negative health outcomes. Therefore, this 10-years retrospective cohort study, aimed to investigate the relationship between shift work and blood and metabolic parameters. This retrospective cohort study was conducted in a metal parts manufacturing industry in 2023. In this study, 204 shift workers and 204 day workers were examined. All the studied blood and metabolic parameters were collected by reviewing the medical records of all participants during a 10-years period (2013-2022). Moreover, the amounts of physical, chemical, and ergonomics harmful agents in the work environment were investigated. All the collected data were analyzed using SPSS version 25.0. The values of Body Mass Index (BMI), Red Blood Cell Count (RBC), Platelets Count (PLT), Thyroid-Stimulating Hormone Level (TSH), Fasting Blood Sugar Level (FBS), Creatinine, Triglyceride (TG), Liver Enzymes level (SGOT and SGPT), and Systolic Blood Pressure (SBP) were higher among the shift work employees, and a significant difference was observed between the values of these parameters between the two groups. The results of logistic regression showed that the highest effect of shift work was observed on the parameters of FBS, TG, SGPT, TSH, Physical activity, BMI, Sleep duration, PLT, and Sleep quality with beta coefficient values of 0.49, 0.33, 0.29, 0.29, 0.20, 0.18, 0.14, 0.13 and, 0.11, respectively (p-value < 0.01). The present study contributes to a growing body of evidence that blood and metabolic factors are likely to be influenced by shift work. These findings have important implications for policy makers, highlighting the need for interventions to mitigate the negative health effects of shift work on workers.

Circulating white blood cell traits and prolonged night shifts: a cross-sectional study based on nurses in Guangxi

This study aimed to elucidate the effects of long day and night shifts on immune cells in a population of nurses. This cross-sectional study in December 2019 was based on a group of nurses. 1568 physically healthy caregivers were included, including 1540 women and 28 men. 1093 nurses had long-term shift work (working in a rotating system for > 1 year). The receiver operating characteristic curve, Ensemble Learning, and Logistic regression analyses were used to evaluate factors related to long-term shift work. The night shift group nurses had significantly higher MPV, PLCR, and WBC and significantly lower BASO%, ELR, MCHC, PLR, RDW-CV, and RDW-SD (P < 0.01). ROC curves showed that WBC, PLR, ELR, RDW_CV, and BASO% were more related to the night shift. Ensemble Learning, combined with the LASSO model, finally filtered out three indicators of night shifts related to ELR, WBC, and RDW_SD. Finally, logistic regression analysis showed that the nurses' night shift situation greatly influenced two peripheral blood ELR and WBC indicators (ELR: log (OR) =  - 3.9, 95% CI: - 5.8- - 2.0; WBC: log (OR) = 0.25, 95% CI: 0.18-0.32). Finally, we showed that, unlike WBC, the relative riskiness of ELR showed opposite results among junior nurses and middle-senior nurses (log (OR) 6.5 (95% CI: 1.2, 13) and - 7.1 (95% CI: - 10, - 3.8), respectively). Our study found that prolonged night shifts were associated with abnormal WBC and ELR, but after strict age matching, WBC remained significantly different. These findings help to confirm that COVID-19 and tumorigenesis (e.g., breast cancer) are significantly associated with circadian rhythm disruption. However, more detailed studies are needed to confirm this.

[Relationship between thyroid function changes with work-/night-shifts and history of thyroid pathology among health personnel]

BACKGROUND

The study aim was to analyze the relationship between functional thyroid pathology and the exposure to work shifts / night shifts, and describe the most prevalent thyroid disorders based on the type of shift.

METHODOLOGY

Cross-sectional study performed in the emergency department of a hospital in Almeria (Spain). Relationships between thyroxine and thyrotropin levels (TSH) and work shifts, professional category and history of thyroid pathology were analyzed.

RESULTS

The study included 133 workers; 80.5% female, average age was 46.11 years (38 - 65), and 52% were part of the nursing staff; thyroid disorders were more frequent in female participants. Most participants (81.2%) had rotating shifts schedules and 11.3% night shifts (12.1% female and 7.7% male). Thyroid alterations were found in 27% of the participants (usually elevated TSH levels and normal thyroxine levels), particularly in those doing night shifts (61.1%). TSH alterations were more frequent in individuals doing night shifts than in rotating shifts (53.3 vs 13.0%; p<0.001). Individuals working night shifts had mean TSH values in the normal range, although significantly higher than the individuals in the rest of the shifts; thyroxine levels were found to be similar. No thyroid disorders were found in day shift participants. Night shift and a history of thyroid pathology were independent predictors of thyroid disorders.

CONCLUSIONS

Night shift schedules and history of thyroid disorders are more frequent in female, both related to the presence of thyroid disorders, indicating the need to include the evaluation of these disorders in health surveillance programs and analyze gender differences.

Association of prolonged occupational co-exposures to electromagnetic fields, noise, and rotating shift work with thyroid hormone levels

The purpose of this study was to determine the association of prolonged occupational co-exposure to extremely low-frequency electromagnetic fields (ELF-EMFs), noise, and rotating shift work with the levels of thyroid hormones (triiodothyronine (T3), thyroxine (T4), and thyroid-stimulating hormone (TSH). From 2016 to 2017, we enrolled all male workers without a history of thyroid disorders and followed them until 2020. To measure ELF-EMFs and noise exposures, we calculated the 8-hour equivalent sound pressure levels (Leq) and the 8-hour average of ELF-EMFs, respectively. Shift work schedules involved 8-hr fixed day and 8-hr clockwise 3-rotating night schedules. The participant's thyroid hormone levels were obtained from blood test results in their medical records. The percentage change in the levels of T3, T4, and TSH was estimated by using different mixed-effects linear regression models. The TSH levels were significantly elevated per a 10-dB increment of noise. The levels of T4 hormone were significantly changed per a unit increase in the levels of ELF-EMFs. Compared to the fixed-day workers, we observed workers exposed to shift work had a significantly lower T4 level. For T4 and TSH hormones, we found significant interactions among noise, ELF-EMFs, and shift work variables. In summary, this study warranted that prolonged exposure to ELF-EMFs, noise, and rotating shift work might be associated with thyroid dysfunction.

Rotating Night Shift Work, Sleep, and Thyroid Cancer Risk in the Nurses' Health Study 2

Night shift work has been associated with breast, prostate, and colorectal cancer, but evidence on other types of cancer is limited. We prospectively evaluated the association of rotating night shift work, sleep duration, and sleep difficulty with thyroid cancer risk in the Nurses' Health Study 2 (NHS2). We assessed rotating night shift work duration (years) at baseline and throughout follow-up (1989-2015) and sleep characteristics in 2001. Cox proportional hazard models, adjusted for potential confounders, were used to calculate hazard ratios (HR) and 95% confidence intervals (CI) for (a) shift work duration, (b) sleep duration, and (c) difficulty falling or staying asleep. We stratified the analyses of night shift work by sleep duration and sleep difficulty. Over 26 years of follow-up, 588 incident cases were identified among 114,534 women in the NHS2 cohort. We observed no association between night shift work and the risk of thyroid cancer. Difficulty falling or staying asleep was suggestively associated with a higher incidence of thyroid cancer when reported sometimes (HR 1.26, 95% CI 0.95, 1.66) and all or most of the time (HR 1.35, 95% CI 1.00, 1.81). Night shift workers (10+ years) with sleep difficulty all or most of the time (HR 1.47; 0.58-3.73) or with >7 h of sleep duration (HR 2.17; 95% CI, 1.21-3.92) had a higher risk of thyroid cancer. We found modest evidence for an increased risk of thyroid cancer in relation to sleep difficulty, which was more pronounced among night shift workers.

Circadian Rhythms in Cardiovascular Function: Implications for Cardiac Diseases and Therapeutic Opportunities

Circadian rhythms are internal 24-h intrinsic oscillations that are present in essentially all mammalian cells and can influence numerous biological processes. Cardiac function is known to exhibit a circadian rhythm and is strongly affected by the day/night cycle. Many cardiovascular variables, including heart rate, heart rate variability (HRV), electrocardiogram (ECG) waveforms, endothelial cell function, and blood pressure, demonstrate robust circadian rhythms. Many experiential and clinical studies have highlighted that disruptions in circadian rhythms can ultimately lead to maladaptive cardiac function. Factors that disrupt the circadian rhythm, including shift work, global travel, and sleep disorders, may consequently enhance the risk of cardiovascular diseases. Some cardiac diseases appear to occur at particular times of the day or night; therefore, targeting the disease at particular times of day may improve the clinical outcome. The objective of this review is to unravel the relationship between circadian rhythms and cardiovascular health. By understanding this intricate interplay, we aim to reveal the potential risks of circadian disruption and discuss the emerging therapeutic strategies, specifically those targeting circadian rhythms. In this review, we explore the important role of circadian rhythms in cardiovascular physiology and highlight the role they play in cardiac dysfunction such as ventricular hypertrophy, arrhythmia, diabetes, and myocardial infarction. Finally, we review potential translational treatments aimed at circadian rhythms. These treatments offer an innovative approach to enhancing the existing approaches for managing and treating heart-related conditions, while also opening new avenues for therapeutic development.

Association between shift work and the risk of hypothyroidism in adult male workers in Korea: a cohort study

Background

Shift work has been reported to have several harmful effects on the human body. However, a small number of studies have evaluated the association between shift work and adverse effects on the thyroid. In our longitudinal study, we examined the causal association between shift work and the risk of hypothyroidism.

Methods

A Kangbuk Samsung Cohort Study was conducted on 112,648 men without thyroid disease at baseline who were followed up at least once between 2012 and 2019. Shift work status and shift schedule types were categorized using standardized questionnaires. Hypothyroidism was defined using the reference ranges of serum thyroid-stimulating hormones and free thyroxine levels. Hazard ratios (HRs) and 95% confidence intervals (CIs) for incident hypothyroidism were estimated using Cox proportional hazards regression analyses with the daytime work group as the reference.

Results

During the 501,237 person-years of follow-up, there were 6,306 incident cases of hypothyroidism (incidence density, 1.26 per 100 person-years). The multivariable-adjusted HR of incident hypothyroidism for the shift work total group that included all shifts compared with the daytime work group was 1.27 (95% CI: 1.15-1.40). For the fixed evening, fixed night, rotating shift, and other shift workers, the multivariable-adjusted HRs (95% CI) were 1.11 (0.76-1.61), 2.18 (1.20-3.93), 1.39 (1.23-1.56), and 1.00 (0.82-1.22), respectively. In subgroup analyses by age, the association between shift work and hypothyroidism was more pronounced in younger participants (< 40 years; HR: 1.31; 95% CI: 1.16-1.47).

Conclusions

Our large-scale cohort study showed an association between shift work and the incidence of hypothyroidism, especially in younger workers with night shifts.

Circadian clock disruption in autoimmune thyroiditis

Objective

A vicious cycle between circadian disruption and escalating immune responses has been described in diverse inflammatory disease. The current study aimed to explore the role of circadian clock disruption in autoimmune thyroiditis (AIT).

Methods

Thirty AIT patients and 30 controls were enrolled and biopsied for thyroid tissues. Alterations of core clock genes expression in AIT thyroid tissues, and its association with serum and tissue inflammatory biomarkers were assessed. For animal studies, C57BL/6J mice administered with porcine thyroglobulin or PBS (as control) combined with adjuvants were sacrificed at four time points to investigate the circadian characteristic of experimental autoimmune thyroiditis (EAT). Light shift (LS) conditions were used to explore the influence of external circadian disturbance on EAT.

Results

The expression of clock genes BMAL1 and PER2 was significantly reduced in thyroid tissues from AIT patients and was negatively correlated to levels of thyroid peroxidase antibodies. In mouse models, diurnal fluctuations of proinflammatory cytokines were demonstrated, and further exposing mice to LS led to overproduction of TNF-α, IFN-γ, and anti-thyroglobulin antibodies. Circadian analysis revealed significant oscillations of Bmal1, Clock, Per2, Cry1, Ror, and Rev-erb, which was broadly disturbed in EAT, LS, and EAT + LS groups.

Conclusions

This study demonstrates that expression pattern of clock genes was disrupted in AIT thyroid, and chronic circadian disruption may aggravate the inflammatory responses in AIT. Whether maintaining a regular circadian rhythm can alleviate autoimmune thyroid diseases warrants further research.

Association of Low Back Pain with Shift Work: A Meta-Analysis

Shift work (SW) is the main working schedule worldwide, and it may cause sleep disorders, breast cancer, and cardiovascular disease. Low back pain (LBP) is a common problem in the workplace; however, the association between LBP and SW remains unclear. Therefore, we conducted a meta-analysis to determine the association between SW and LBP. This study was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. The PubMed, Embase, and Web of Science databases using a set of associated keywords were queried. The inclusion criteria were as follows: (1) adult employees hired by a company or organization; (2) SW exposure; and (3) the outcome of LBP according to examination or assessment. A total of 40 studies were included that met the inclusion criteria for the meta-analysis. SW was significantly associated with LBP (odds ratio [OR]: 1.31, 95% confidence interval [CI]: 1.18−1.47, p < 0.00001). Furthermore, it was observed that LBP was significantly associated with night shift (NS) (OR: 1.49, 95% CI: 1.24−1.82, p < 0.0001) but not with rotating shift (RS) (OR: 0.96, 95% CI: 0.76−1.22, p = 0.49). Moreover, LBP was significantly associated with SW in health care workers (HCWs) (OR: 1.40, 95% CI: 1.20−1.63, p < 0.0001) but not in non-HCWs (OR: 1.19, 95% CI: 0.94−1.50, p = 0.14). SW was significantly associated with LBP. Furthermore, the subgroup analysis showed that NS, but not RS, was associated with LBP. Compared with SW in non-HCWs, SW in HCWs was significantly associated with LBP.

Shift work is significantly and positively associated with dementia: A meta-analysis study

Background

Shift work may disrupt the sleep and wake cycles and negatively affect physical and mental health. Dementia is a neurodegenerative disorder with progressively declining cognition that is receiving increasing attention. Studies on the association between shift work and dementia are rare. Herein, we conducted a meta-analysis to investigate the association between shift work and dementia.

Materials and methods

This study was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. We searched the PubMed, Embase, and Web of Science databases using a related set of keywords. The inclusion criteria were as follows: (1) adult employees working in a factory, company, or organization; (2) exposure to shift work/non-shift work; and (3) outcome of dementia based on examination or assessment. A meta-analysis using a fixed-effects model was performed. The hazard ratio of dementia was compared between shift workers and non-shift workers.

Results

Five studies were included in the quantitative synthesis, and two were selected for further meta-analysis. A random-effects model showed a modest association between shift work and an increase in dementia cases (pooled hazard ratio = 1.13; 95% confidence interval: 1.04-1.23; p = 0.04). This association also occurred in night workers for more than 1 year.

Conclusion

Shift work and long-term night work were modestly associated with a higher risk of developing dementia. Avoiding long-term night shifts may be effective in reducing dementia risk. Further studies are required to confirm this hypothesis.

Shift work is significantly and positively associated with possible gastro-esophageal reflux disease: A meta-analysis study

Background

One of the health issues related to shift work patterns is possible gastro-esophageal reflux disease (GERD) symptoms. However, the association between shift work and possible GERD symptoms through meta-analysis has not been developed in the current literature field. Therefore, the purpose of this study is to analyze the association between shift work and possible GERD symptoms through meta-analysis.

Methods

Studies containing target keywords were found in three datasets, and four articles were selected for further analysis after examining the title, abstract, and text. All prevalence odds ratios (ORs) among different groups of the population and the standard error (SE) from each included study were calculated for conducting meta-analysis.

Result

The pooled OR has shown a significant positive association between shift work and possible GERD (OR 1.53; 95% confidence interval [CI] 1.33-1.77, p-value 0.003). Compared to non-shift workers, the subgroup analysis indicates there are positive associations between possible GERD and the night shift (OR 1.39; 95% CI 1.16-1.66), and the rotating shift (OR 1.83; 95% CI 1.44-2.33). The subgroup analysis has also shown similar trends in shift working men (OR 1.28; 95% CI 1.03-1.60) and shift workers of both genders (OR 1.75; 95% CI 1.45-2.11).

Conclusion

This study has shown a positive association between shift work and possible GERD.

Epidemiology of neurodegenerative diseases in the East African region: A meta-analysis

INTRODUCTION

There is a scarcity of epidemiological data on neurodegenerative diseases (NDs) in East Africa. This meta-analysis provides the regional prevalence of NDs, their contributing factors, and evidence of change over time concerning gender per age or year.

METHODS

Articles were retrieved from electronic databases following the PRISMA standard.

RESULTS

Forty-two studies were reviewed, and 25 were meta-analyzed with a random-effects model. The pool estimate proportion of 15.27%, 95% CI (0.09-0.23) (I² = 98.25%), (Q = 1,369.15, p < 0.0001) among a population of 15,813 male/female and 1,257 with NDs. Epidemiological characteristics associated with NDs include Dyskinesias prevalence 55.4%, 95% CI (13.5; 90.9), I² (96%) and subsistence farming prevalence 11.3%, 95% CI (5.8; 20.9), I² (99%). Publication bias by Egger test was (z = 4.1913, p < 0.0001), while rank correlation test using Kendall's model was (tau = 0.1237, p = 0.3873). Heterogeneity (R² design = 5.23%, p design < 0.0001; R² size = 52.163%, p size < 0.001; and R² period = 48.13, p period < 0.0001. Covariates (R² design + size + period = 48.41%, p < 0.001).

CONCLUSION

There is a high prevalence of NDs in the East African region, which could impact life expectancy, morbidity, and quality of life. Thus, early screening and regular surveillance could assist in management strategies.

Farklı sektörlerde çalışanlarda tiroid hormon düzeylerini etkileyen işle ilgili faktörler

Aim: This study purposed to evaluate thyroid hormone levels and work-related factors in workers. Materials and Methods: This retrospective study was perfomed at 475 workers who applied to the outpatient clinic of occupational diseases between November 2015-February 2020. Sociodemographic variables, work-related factors and laboratory records were retrospectively evaluated. Student’s t-test was performed to compare mean values. Multiple linear regression analysis was used, including independent variables that were found to be statistically significantly related in binary analysis of thyroid-stimulating hormone (TSH) value. Results: Of the 475 participants, 433 (91.2%) were male and 173 (36.4%) worked night shifts, 293 (61.7%) were exposed to silica dust, 23 (4.8%) to plastic fumes, and 80 (16.8%) to solvent. Study participants who were over 40 years of age, night shift workers, and workers with exposure to silica dust had significantly lower TSH values (p=0.026, p=0.023, p=0.002, respectively); male workers, workers under 40 years of age, and workers with body mass index (BMI)

Night Shift Work Is Associated with Reduced Rate of Humoral Response Following Vaccination for HBV

Night shift work has been associated with cardiovascular and metabolic disease, endocrine and immunological disorders. Published studies have reported that a reduced total sleep time with sleep-wake cycle alterations were associated with a reduced rate of humoral response following vaccination. Our study aimed to evaluate the association between night shift work and serological status for HBV among workers employed in a university hospital in Rome. We evaluated medical records of 986 HCWs working at Tor Vergata Policlinic of Rome. We screened all study subjects for anti-HBs IgG, anti-HBc IgG and HBsAg. Serological protection for HBV was evaluated in relation to sex, age group, job task, risk setting and night shift work status. Protective titer was found in 856 (86.8%) study participants and the mean titer was significantly high in females, in subjects aged less than 40 years, in night shift workers and in high-risk setting workers. After adjustment for study covariates, night shift work was no longer associated with an HBV-protective titer. This finding suggests that a vaccination strategy for dampening HBV transmission should be carefully addressed in health care workers (HCWs) doing night shift.

Circadian Rhythm Effects on the Molecular Regulation of Physiological Systems

Nearly every system within the body contains an intrinsic cellular circadian clock. The circadian clock contributes to the regulation of a variety of homeostatic processes in mammals through the regulation of gene expression. Circadian disruption of physiological systems is associated with pathophysiological disorders. Here, we review the current understanding of the molecular mechanisms contributing to the known circadian rhythms in physiological function. This article focuses on what is known in humans, along with discoveries made with cell and rodent models. In particular, the impact of circadian clock components in metabolic, cardiovascular, endocrine, musculoskeletal, immune, and central nervous systems are discussed. © 2021 American Physiological Society. Compr Physiol 11:1-30, 2021.

The association between shift work and possible obstructive sleep apnea: a systematic review and meta-analysis

BACKGROUND

Shift work is a workschedule, since industrial era and some employees work in shift. It causes a desynchronization of the biological clock with consequences on sleep amount and quality, such as insomnia and easy fatigue. Obstructive sleep apnea (OSA) is one of the sleep problems that are getting more and more attention, but studies on the association between shift work and OSA were rare. Herein, we aimed to conduct a systematic review and meta-analysis to investigate the association between shift work and possible OSA.

METHODS

This study was conducted according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. We queried PubMed, Embase, and Web of Science databases using a related set of keywords. The inclusion criteria were as follows: (1) participants were adult employees hired by a company or organization; (2) exposure was shift work; and (3) outcome was possible OSA according to examination or assessment.

RESULTS

We included six studies in the systematic review and five studies were selected for further meta-analysis. A random-effects model showed an association of shift work with a small, non-significant increase in possible OSA cases (pooled prevalence relative risk = 1.05; 95% CI 0.85-1.30; p = 0.65). This association occurred in both healthcare and non-healthcare workers group.

CONCLUSION

The association between shift work and possible OSA remains inconclusive and could be small if not negligible. Future studies should assess the association between specific work schedules and specific OSA definitions.

TRIAL REGISTRATION NUMBER

PROSPERO ID: CRD42020156837.

The Relationship Between Night Shift Work and the Risk of Abnormal Thyroid-Stimulating Hormone: A Hospital-Based Nine-Year Follow-up Retrospective Cohort Study in Taiwan

Background

Health-care providers typically undergo shift work and are subjected to increased stress. Night shift work may induce disturbed sleep cycles and circadian rhythm. The objective of this study was to explore if night shift workers (NSWs) show an increased risk of abnormal thyroid-stimulating hormone (TSH).

Methods

We conducted a retrospective cohort study of 574 employees without thyroid disease and abnormal TSH at baseline who underwent annual check-ups between 2007 and 2016 in a medical center. NSWs were defined as those with working time schedules other than daytime hours. We calculated the incidence rate and estimated the adjusted hazard ratio (HR) for incident abnormal TSH and subclinical hypothyroidism compared with non-NSWs using a Cox regression model.

Results

A total of 56 incident abnormal TSH cases and 39 subclinical hypothyroidism cases in NSWs were identified during 3000 person-years of follow-up. In models adjusted for age, sex, obesity, and working departments, we found no increased relative risk for incident abnormal TSH (HR: 0.72, 95% confidence interval: 0.33–1.60) or subclinical hypothyroidism (HR: 0.52, 95% confidence interval: 0.19–1.45) when comparing NSWs to non-NSWs; nor were incidence rates significantly different among exclusively medical employees after excluding administrative staff.

Conclusion

In this hospital-based nine-year follow-up retrospective cohort study, NSWs were not associated with increased relative risk of incident abnormal TSH and subclinical hypothyroidism, in contrast to previous cross-sectional studies.

Chronothyroidology: Chronobiological Aspects in Thyroid Function and Diseases

Chronobiology is the scientific discipline which considers biological phenomena in relation to time, which assumes itself biological identity. Many physiological processes are cyclically regulated by intrinsic clocks and many pathological events show a circadian time-related occurrence. Even the pituitary–thyroid axis is under the control of a central clock, and the hormones of the pituitary–thyroid axis exhibit circadian, ultradian and circannual rhythmicity. This review, after describing briefly the essential principles of chronobiology, will be focused on the results of personal experiences and of other studies on this issue, paying particular attention to those regarding the thyroid implications, appearing in the literature as reviews, metanalyses, original and observational studies until 28 February 2021 and acquired from two databases (Scopus and PubMed). The first input to biological rhythms is given by a central clock located in the suprachiasmatic nucleus (SCN), which dictates the timing from its hypothalamic site to satellite clocks that contribute in a hierarchical way to regulate the physiological rhythmicity. Disruption of the rhythmic organization can favor the onset of important disorders, including thyroid diseases. Several studies on the interrelationship between thyroid function and circadian rhythmicity demonstrated that thyroid dysfunctions may affect negatively circadian organization, disrupting TSH rhythm. Conversely, alterations of clock machinery may cause important perturbations at the cellular level, which may favor thyroid dysfunctions and also cancer.

Reinterpreting patterns of variation in human thyroid function: An evolutionary ecology perspective

Two hundred million people worldwide experience some form of thyroid disorder, with women being especially at risk. However, why human thyroid function varies between populations, individuals, and across the lifespan has attracted little research to date. This limits our ability to evaluate the conditions under which patterns of variation in thyroid function are best understood as 'normal' or 'pathological'. In this review, we aim to spark interest in research aimed at understanding the causes of variation in thyroid phenotypes. We start by assessing the biomedical literature on thyroid imbalance to discuss the validity of existing reference intervals for diagnosis and treatment across individuals and populations. We then propose an evolutionary ecological framework for understanding the phylogenetic, genetic, ecological, developmental, and physiological causes of normal variation in thyroid function. We build on this approach to suggest testable predictions for how environmental challenges interact with individual circumstances to influence the onset of thyroid disorders. We propose that dietary changes, ecological disruptions of co-evolutionary processes during pregnancy and with pathogens, emerging infections, and exacerbated stress responses can contribute to explaining the onset of thyroid diseases. For patients to receive the best personalized care, research into the causes of thyroid variation at multiple levels is needed.