BACKGROUND
Hypothyroidism is common in the US adult population and is treated with thyroid hormone therapy, specifically levothyroxine. Indeed, levothyroxine is among the most commonly prescribed medications in the US. While many patients with hypothyroidism will only have that problem, most have other chronic illnesses. The presence of two or more chronic illnesses at the same time in one person is referred to as multimorbidity. One can study the relationships between different chronic conditions using a computer programs that develop a “network” analysis, creating a visual diagram with “nodes” (the chronic conditions) and “edges” (the lines that connect the nodes). The purpose of this study was to evaluate the relationships, positive or negative, of different illnesses with respect to hypothyroidism.

THE FULL ARTICLE TITLE
Moratalla-Navarro F et al 2020 MorbiNet study: Hypothyroidism comorbidity networks in the adult general population. J Clin Endocrinol Metab. Epub 2020 Dec 15. PMID: 33319230.

SUMMARY OF THE STUDY
The authors used a registry of electronic health records from Catalonia, Spain. For their analysis, they included data of a little over 3 million adults living in that area with multiple chronic illnesses. Almost 10% of these individuals had hypothyroidism. They analyzed these data by age and sex and they calculated the chances of having multiple chronic illnesses in patients with and without hypothyroidism. The results were presented as odds ratios (OR). If the OR was higher than 1.2, they considered the relationship significant. If the OR was less than 0.8, they considered the relationship negative or non-significant.

In their network-based analysis, the authors found a significant positive relationship between hypothyroidism and respiratory cancers (OR 2.5), birth disorders (OR 2.2), thyroid cancer (OR 2.1), hyperthyroidism (OR 1.6) and type 1 Diabetes (OR 2.13). These associations were found to be stronger in men than women. Other associations included irregular heart rhythms, enlarged thyroid and cholesterol disorders.

Other conditions were negatively associated, meaning that were less likely to co-occur in patient with hypothyroidism: HIV/AIDS (OR 0.7), tobacco abuse (OR 0.7), and, for women in particular, chronic alcohol use and lung cancer.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This study shows that hypothyroidism is frequently associated with multiple other medical conditions. Some of these associations are not unexpected since they are either indirectly leading to hypothyroidism (for example, thyroidectomy after thyroid cancer or hyperthyroidism) or sharing autoimmunity as their cause (for example, type 1 diabetes). Multimorbidity can increase the health burden of the individual and particularly for hypothyroidism, it can make the management more complicated. It is important to be aware of these associations in patients with hypothyroidism.

— Susana Ebner

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