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Letter
Thyroxin substitution and open-angle glaucoma: a nationwide register-based study on filled prescriptions
  1. Jan Calissendorff1,2,3,
  2. Rickard Ljung2,4,5
  1. 1Metabolism and Diabetology, Department of Endocrinology, Karolinska University Hospital, Stockholm, Sweden
  2. 2Department of Molecular Medicine and Surgery, Karolinska Institutet, Stockholm, Sweden
  3. 3Department of Medicine, Endocrine Section, Mälarsjukhuset, Eskilstuna, Sweden
  4. 4The National Board of Health and Welfare, Stockholm, Sweden
  5. 5Equity and Health Policy, Department of Public Health Sciences, Karolinska Institutet, Stockholm, Sweden
  1. Correspondence to Rickard Ljung, The National Board of Health and Welfare, Rålambsvägen 3, SE 106 30 Stockholm, Sweden; rickard.ljung{at}socialstyrelsen.se

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The issue of the possible association between hypothyroidism and glaucoma is well studied but still remains controversial. Some previous studies have found a correlation between hypothyroidism and open-angle glaucoma1–3 whereas others have not.4–6 Our aim was to test the hypothesis that hypothyroidism is associated with open-angle glaucoma in a large nationwide register-based cohort study on filled prescriptions, a design unique to the existing literature on the subject.

All 2 088 297 residents in Sweden aged 60–89 years who were alive during the period 31 December 2007 to 31 December 2008 constituted our source population. We used the unique personal identity number, assigned to each Swedish resident (about 9 million in total), to link information from three population-based registers of almost complete coverage and high quality—the Prescribed Drug Register, the Cause of Death Register and the Register of the Total Population.

The Swedish Prescribed Drug Register contains all dispensed prescriptions in Sweden. Before 1 July 2005, the personal identity number was not recorded; hence, register linkage is not possible for prescriptions dispended before this date. The register contains data on substance, amount dispensed, dosage, expenditure and reimbursement as well as the age, sex and place of residence of the patient. The register lacks information on diagnosis or indication for treatment.

We excluded subjects on antiglaucoma medication (Anatomical Therapeutic Chemical (ATC) S01E) or on antithyroid preparation drugs (ATC H03B) in 2007. Outcome was defined as having at least one dispensed prescription of antiglaucoma preparation in 2008 (ATC S01E). Prescriptions dispensed between 1 January 2007 and 31 December 2007 gave exposure status. Those who had at least one dispensed prescription of thyroid preparations (ATC H03A) were classified as subject to thyroxin substitution. Marital status was defined as either married or not married. Estimates of risk of antiglaucoma medication were calculated as ORs with 95% CIs, using unconditional logistic regression analysis.

There was an expected female predominance among thyroxin substitution users and they were also older than the general population (table 1). Table 2 presents the OR estimates for the association between thyroxin substitution and antiglaucoma medication. The OR adjusted for age, sex and marital status shows no association between thyroxin substitution and antiglaucoma medication (OR 1.00, 95% CI 0.95 to 1.06). In the stratified analysis, the OR is close to unity for women and slightly increased for men.

Table 1

Characteristics of the 2 088 297 residents according to their thyroid medication in 2007 (N,%)

Table 2

Thyroxin substitution and the OR with 95% CIs of antiglaucoma medication, for Swedish subjects free of antiglaucoma treatment in 2007

This study found no evidence of an association between hypothyroidism and the risk of developing open-angle glaucoma.

The main strengths of the present study include the large sample size and the complete nationwide coverage of all filled prescriptions of thyroxin substitution and antiglaucoma medication. The nationwide register-based design counteracts recall or selection bias. However, there are weaknesses that need to be discussed. Since the design is based on filled prescriptions, we only capture patients with hypothyroidism or glaucoma who have been treated and had their prescriptions filled. Hence, undiagnosed cases of hypothyroidism and open-angle glaucoma could be hidden in the group with neither use of these pharmacological preparations. With rare exceptions, thyroxin is only prescribed to treat hypothyroidism, although it can be used as an adjuvant in psychiatric diseases. In our study, all types of patients with hypothyroidism are included, and a weakness is that we cannot control for how many of our subjects are treated due to a milder or more long-standing disease. As for antiglaucoma treatment there is no other known indication.

In conclusion, this nationwide study of more than 2 million elderly persons found no correlation between those treated with antiglaucoma preparations and those on thyroxin substitution. This finding does not suggest that there is an association between the two diseases.

References

Footnotes

  • Funding RL was partly supported by grants from the Astrid and David Hagelén Foundation and from the Swedish Research Council (SIMSAM).

  • Competing interests None.

  • Ethics approval The Swedish National Board of Health and Welfare is a government agency and may, in accordance with Swedish law, use population-based registers to follow and analyse health and social conditions among the general population. Data were made available to us in such a way that individuals could not be identified.

  • Provenance and peer review Not commissioned; externally peer reviewed.