Article - A systematic review and meta-analysis investigating the relationship between metabolic syndrome and the incidence of thyroid diseases.

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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

A: Meta-analyses, position-stands, randomized-controlled trials (RCTs)

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, I = 0%). Pooled analysis was not possible for subclinical hypothyroidism, due to large heterogeneity (I = 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, I = 0%) and subclinical hypothyroidism (RR 1.50 (1.05-1.94), I = 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), I = 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.