Age-related macular degeneration (AMD) results from gradual damage to the cells of the macula. Common in people over 60-years-old, the disease is characterized by drusen, pigmentary changes, choroidal neovascularization and geographic atrophy, and can lead to irreversible blindness.
A lot of research has been done on AMD, including its associations with other diseases. In animal models and in epidemiological research, the association of thyroid hormone levels with AMD has been studied – however, the relationship between thyroid cancer and AMD has been less clear.
To determine if thyroid cancer is a risk factor for AMD, a study was conducted in Taiwan on the association between AMD and thyroid cancer in the older population. The study included patients ≥50-years-old who were diagnosed with thyroid cancer between 2000 and 2008. The patients were identified from the country’s National Health Insurance Research Database (NHIRD), which contains the medical claims data of more than 22 million people in Taiwan, with a coverage rate of over 98 percent. The National Health Insurance Administration (NHIA) strictly monitors and checks the insurance claims for repayment to prevent healthcare fraud. Thus, the diagnosis and procedure codes in the NHIRD are reliable.
The study involved 5,253 thyroid cancer patients and 21,012 age- and gender-matched controls. Time-varying Cox proportional hazard models were used to evaluate the association between thyroid cancer and AMD. The multivariable models encompassed conventional cardiovascular risk factors, hypothyroidism, hyperthyroidism, vitreous floaters, myopia and treatment modality of thyroid cancer. The analysis process was stratified by age, gender, and comorbidity. The comorbidities in the study were diabetes, hypertension, hyperlipidemia, coronary artery disease, high myopia, hyperthyroidism, hypothyroidism, asthma, chronic obstructive coronary disease, stroke and tobacco dependency.
The study found that thyroid cancer patients older than 50 have an increased risk of developing AMD.
The thyroid cancer cohort consisted of 24.5% men and the median age was 59.1 years (53.7–67.4 years). The AMD incidence reported was 40.7 per 10,000 persons per year. They found that the thyroid cancer cohort had a higher risk of AMD than the non-thyroid cohort, especially the male patients and patients with comorbidities.
This study on the association between AMD and thyroid cancer in the aged population was the first retrospective and largest cohort study. It found that thyroid cancer patients exhibited a 1.38-fold higher risk of AMD than control subjects.
Thyroid cancer patients aged over 65 exhibited a higher adjusted risk (1.52-fold) after stratification by age, compared with the controls across all age stratifications. This association is of concern for the elderly, thus close monitoring of vision impairment and retina conditions is necessary.
The researchers believe their findings associating thyroid cancer with AMD have several clarifications.
Firstly, thyroid cancer and AMD have some common etiologies, including hyperinsulinemia state, obesity and stimulation of thyroid growth. In fact, obesity and hyperinsulinemia state have been linked to an increased risk of thyroid cancer. Considering that the thyroid cancer patients in the study had a higher prevalence of diabetes, hypertension, hyperlipidemia and coronary artery disease, the researchers suggested that the hyperinsulinemia state and oxidative stress cause the development of thyroid cancer as well as of RPE dysfunction and AMD. This would explain why the thyroid cancer patients examined had an increased risk of AMD.
Secondly, mitochondrial dysfunction would be another mechanism for the higher risk of
AMD in thyroid cancer patients. Mitochondrial dysfunction plays a common role in metabolic diseases, cancer and degenerative disease. Tumour-specific markers of somatic mitochondrial DNA mutations have been identified in thyroid tumours. However, more research needs to be conducted to investigate whether thyroid cancer and AMD have common pathogenic pathways involving genetic or somatic mutations. It’s important to determine the extent to which these two diseases could be reversed or modulated.
According to the researchers, the study showed that thyroid cancer patients, especially males and those with comorbidities, had higher risk of AMD, suggesting that comorbidities and male sex may contribute to the development of AMD. Although a clear association between the risk of AMD and gender has not been determined, clinical attention is needed for these specific groups.
Reference:
Lin SY, Hsu WH, Lin CL, et al. Evidence for an Association between Macular Degeneration and Thyroid Cancer in the Aged Population. Int J Environ Res Public Health. 2018;15(5).
