Thyroid disorders and glaucoma

Practitioners urged to keep an eye toward glaucoma in thyroid cases

Patients with thyroid conditions are more apt to have glaucoma than those without the disorder, according to results of a recent study published in the November 2008 issue of the online British Journal of Ophthalmology. Investigators looked at a possible link between thyroid disorders and glaucoma by prior research, said Christopher A. Girkin, M.D., professor of ophthalmology and director, Glaucoma Service, University of Alabama at Birmingham, Birmingham.
“Back in 2004 the same group published a paper in Ophthalmology looking at an administrative data set from the local veteran’s hospital, which consisted of about 5,000 to 6,000 male patients,” Dr. Girkin said. This study showed a greater risk of glaucoma for those with pre-existing hypothyroidism than for controls. Likewise, investigators were spurred by other smaller studies in the literature that suggested that there might be an association.

Making the thyroid connection

In this most recent population-based study of 12,376 participants from a 2002 National Health Interview Survey, individuals were asked if they had ever been told that they had thyroid problems, according to Gerald McGwin, Ph.D., professor, Department of Epidemiology, University of Alabama at Birmingham.
“What we found was that thyroid disorders may increase the risk of glaucoma,” Dr. McGwin said. “We found that the prevalence of glaucoma was about 40% higher in those who reported thyroid problems.”
In the study, 11.9% of patients reported a history of thyroid problems and there was an overall prevalence of glaucoma at 4.6%. “We happened to find a fairly significant association with thyroid with the incidence of glaucoma,” Dr. Girkin said. “This association was strong even when adjusted for other comorbid conditions.”
Investigators theorize that there may be several different mechanisms at play. “It may be that hypothyroidism may diminish outflow in the eye,” Dr. Girkin said. “There is some preliminary evidence that potentially treating hypothyroidism can improve outflow facility.”

The clinical case

Clinically speaking this could turn out to be a helpful marker for patients, believes Dr. McGwin. “I think that if we could replicate it and maybe narrow it down to hypo- or hyper-, what it suggests is that people who have a history of thyroid disease may want to see their ophthalmologists more regularly. If they’re seeing one regularly, they should make sure that their pressures are checked,” he said. “It does become apparent that while there is no way to reverse glaucoma, it can be treated and treated early.”
Ophthalmologists might likewise want to take note of those who have the condition. “For general eye care patients, I would think that a thyroid condition may heighten their suspicion for glaucoma,” Dr. McGwin said.
Dr. Girkin thinks that the results of this study coupled with those in the literature point to a strong likelihood of a clinical link between thyroid disease and glaucoma. “I think that based upon the overall literature, not just this one study, thyroid disease may be associated with a higher risk of glaucoma,” he said. “I think that it should be considered a tentative risk factor.”
There are several tentative risk factors already out there, however, he points out. “At least right now there’s not as much evidence as for a lot of other risk factors like central corneal thickness, [intraocular] pressure, and a strong family history,” he said. “I think those are stronger risk factors, or at least there is more evidence for those risk factors.”
However, the study does need replication. “There are a lot of weaknesses in a study like this,” Dr. Girkin said. “The use of self-reported diagnosis isn’t as good; it probably weakens the association between glaucoma and thyroid disease because there is a lot of non-differential misclassification.”
Despite this, Dr. Girkin sees the study as playing a significant role. “These studies are important in that they’re surveys of the general population and could point out directions for future research,” he said. “It does show a fairly strong association adjusted for many covariats and suggests, along with the previous literature, that hypothyroidism, the most common type of thyroid condition, may be associated with glaucoma,” Dr. Girkin said. “I think now we need to look at the effect of thyroid disease and the effect of treating it on glaucoma patients prospectively to determine if there’s an true association and if treatment influences the outcome.”

Editors’ note: Dr. Girkin has financial interests with Alcon (Fort Worth, Texas), Allergan (Irvine, Calif.), Carl Zeiss Meditec (Dublin, Calif.), Heidelberg Engineering (Vista, Calif.), and Pfizer (New York), among others. Dr. McGwin has no financial interests related to his comments.

Contact information

Girkin: 205-325-8110, cgirkin@uab.edu
McGwin: 205-325-8117, Gerald.McGwin@ccc.uab.edu