Plain language summary
Metabolic syndrome (MetS) is a cluster of conditions, including obesity, hypertension, dyslipidaemia, and insulin resistance, that increase the risk of cardiovascular diseases and type 2 diabetes. Thyroid diseases, such as hypothyroidism and hyperthyroidism, have been suggested to be linked with MetS due to their influence on metabolic processes. The primary aim of this study was to systematically review and analyse the association between metabolic syndrome and the incidence of thyroid diseases, including hypothyroidism, hyperthyroidism, and thyroid nodules. This research was a systematic review and meta-analysis, incorporating data from multiple observational studies and clinical trials that examined the relationship between MetS and thyroid diseases. Results did not show any clear evidence on the association between MetS at baseline and incidence of overt or subclinical hypothyroidism. However, there was an association between obesity and both overt and subclinical hypothyroidism. Authors concluded that their findings indicate an association between obesity at baseline and the incidence of hypothyroidism. Additionally, larger studies as well as individual participant data meta-analyses that standardise definitions and statistical methods are needed to help elucidate these associations.
Expert Review
Conflicts of interest:
None
Take Home Message:
- Obesity is associated with an increased risk of overt and subclinical hypothyroidism.
- Maintaining a healthy body weight may reduce the risk of hypothyroidism.
Evidence Category:
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X
A: Meta-analyses, position-stands, randomized-controlled trials (RCTs)
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B: Systematic reviews including RCTs of limited number
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C: Non-randomized trials, observational studies, narrative reviews
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D: Case-reports, evidence-based clinical findings
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E: Opinion piece, other
Summary Review:
Introduction
Metabolic syndrome (MetS) increases an individual's risk of developing cardiovascular disease and type 2 diabetes. Thyroid disorders have also been linked to MetS. This systematic review and meta-analysis aimed to investigate the association between MetS, and its components, and incidence of thyroid disorders.
Methods
- The study adhered to the PRISMA statement.
- 4 outcomes were investigated: overt hypothyroidism, subclinical hypothyroidism, overt hyperthyroidism and subclinical hyperthyroidism.
- 8 studies were included in the qualitative synthesis.
- 7 studies were included in the quantitative synthesis (n=153, 237).
Results
- The pooled unadjusted odds ratio (OR) for the association between MetS and incidence of overt hypothyroidism was 0.78 (0.52–1.16).
- Pooled unadjusted OR from 3 studies for the association between diabetes mellitus and the incidence of overt hypothyroidism was 0.83 (0.37-1.86).
- The pooled OR from 2 studies for the association between prediabetes and overt and subclinical hypothyroidism was 0.87 (0.50–1.52) and 1.01 (0.89–1.15), respectively.
- 1 study reported the association between abdominal obesity and subclinical hypothyroidism (adjusted Hazard Ratio (HR): 1.07, 0.93–1.25) and overt hypothyroidism (unadjusted OR: 1.32 (0.72–2.42).
- 2 studies assessed the association between obesity at baseline and overt (pooled RR: 3.10, 1.56–4.64) and subclinical hypothyroidism (pooled RR 1.50, 1.05–1.94).
- The association between hypertension and overt (Relative Risk (RR): 1.68, 1.53–1.84) and subclinical hypothyroidism (adjusted HR: 1.24, 1.04–1.48) were reported in 1 study each.
- 1 study reported an association between hypertriglyceridemia and overt hypothyroidism (RR: 1.79, 1.15–2.79) and high total cholesterol and subclinical hypothyroidism (RR: 1.60, 1.15–2.23).
- 1 study found an association between hypertriglyceridemia and an increased risk of subclinical hypothyroidism (adjusted HR: 1.18, 1.00–1.39).
Conclusion
MetS was not associated with overt or subclinical hypothyroidism. Obesity was positively associated with overt and subclinical hypothyroidism.
Clinical practice applications:
- The link between MetS and its components and thyroid disease is inconclusive.
- Obesity was shown to be associated with overt and subclinical hypothyroidism.
Considerations for future research:
- Obesity was found to be associated with an increased risk of overt and subclinical hypothyroidism. Further studies exploring the mechanism for obesity to disrupt thyroid hormones may inform clinical interventions and offer insights into potential preventative treatments for hypothyroidism.
- Studies investigating the impact of weight loss interventions on TSH levels in obese individuals with overt and subclinical hypothyroidism are warranted to explore the potential benefit of weight loss in the management of hypothyroidism.
Abstract
PURPOSE To assess the prospective association between metabolic syndrome (MetS), its components, and incidence of thyroid disorders by conducting a systematic review and meta-analysis. METHODS A systematic search was performed in Ovid Medline, Embase.com, and Cochrane CENTRAL from inception to February 22, 2023. Publications from prospective studies were included if they provided data on baseline MetS status or one of its components and assessed the incidence of thyroid disorders over time. A random effects meta-analysis was conducted to calculate the odds ratio (OR) for developing thyroid disorders. RESULTS After full-text screening of 2927 articles, seven studies met our inclusion criteria. Two of these studies assessed MetS as an exposure (N = 71,727) and were included in our meta-analysis. The association between MetS at baseline and incidence of overt hypothyroidism at follow-up yielded an OR of 0.78 (95% confidence interval [CI]: 0.52-1.16 for two studies, I2 = 0%). Pooled analysis was not possible for subclinical hypothyroidism, due to large heterogeneity (I2 = 92.3%), nor for hyperthyroidism, as only one study assessed this association. We found evidence of an increased risk of overt (RR: 3.10 (1.56-4.64, I2 = 0%) and subclinical hypothyroidism (RR 1.50 (1.05-1.94), I2 = 0%) in individuals with obesity at baseline. There was a lower odds of developing overt hyperthyroidism in individuals with prediabetes at baseline (OR: 0.68 (0.47-0.98), I2 = 0%). CONCLUSIONS We were unable to draw firm conclusions regarding the association between MetS and the incidence of thyroid disorders due to the limited number of available studies and the presence of important heterogeneity in reporting results. However, we did find an association between obesity at baseline and incidence of overt and subclinical hypothyroidism.
Methodological quality
Jadad score
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Allocation concealment
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