Year : 2014 | Volume
: 11 | Issue : 1 | Page : 4--5
Hypothyroidism and glaucoma: Is there an association?
Vasan Eye Clinic, Kothrud, Pune, Maharashtra, India
Vasan Eye Clinic, Kothrud, Pune, Maharashtra
|How to cite this article:|
Bhat S. Hypothyroidism and glaucoma: Is there an association?.Thyroid Res Pract 2014;11:4-5
|How to cite this URL:|
Bhat S. Hypothyroidism and glaucoma: Is there an association?. Thyroid Res Pract [serial online] 2014 [cited 2019 Jul 15 ];11:4-5
Available from: http://www.thetrp.net/text.asp?2014/11/1/4/124185
Glaucoma is an important cause of blindness world-wide, as a rise in intraocular pressure is associated with optic disc damage. Recently, a large study had indicated an association between hypothyroidism and glaucoma.  Hypothyroidism is also a common disease.  This article aims to put the issue of hypothyroidism and its link to glaucoma in perspective for the clinician treating this illness.
In a large cross-sectional study of over 12,376 subjects in the USA, authors reported an association between the hypothyroidism and glaucoma.  This was a population based study, as part of the large National Health Interview Survey. The authors report that about 4.6% of subjects reported glaucoma, while 11.9% of subjects reported a thyroid disorder. Among patients with who reported thyroid disease, 6.5% also reported glaucoma. And, among patients who did not report thyroid disease, only 4.4% reported glaucoma. This difference was statistically significant and remained so despite adjusting for various confounding factors, like for instance, age as well as gender and smoking with an odds ratio of 1.38 in the study. Although the results do not directly implicate hypothyroidism, as hypothyroidism is arguably among the most common of thyroid problems, this association reported by Cross et al., deserves critical study. 
Why is there an association between hypothyroidism and glaucoma? Well, it has been postulated by Smith et al., as early as in 1993, that hypothyroidism causes the deposition of hyaluronic acid and related materiel in the trabecular mesh work of the eye, this raising intraocular pressures.  The same group also reported that the prevalence of open angle glaucoma in subjects with hypothyroidism was high as high as 23%. However, this association has been questioned in other studies. Interestingly, Centanni et al., reported that the intra-ocular pressures (IOP) may be elevated even in patients with subclinical hypothyroidism.  The authors also report that the use of levothyroxine reduced the raised IOP seen in subjects with subclinical hypothyroidism. 
Although the data has been conflicting, the question that clinicians would be most interested in knowing would be this given the association between hypothyroidism in glaucoma, should doctors actively screen for hypothyroidism in subjects with glaucoma. Well, this clinical question was recently addressed in a study published from Greece.  The authors selected 78 subjects with primary open angle glaucoma and also took a carefully selected control group. Thyroid function tests were carried out in both groups and the authors report no statistically significant differences in thyroid function parameters or the prevalence of hypothyroidism. This led the authors to suggest that there seems to be no rationale for actively screening patients with glaucoma for hypothyroidism.
What about the role of screening for glaucoma in subjects with hypothyroidism? While the previous studies have reported a high prevalence, as discussed earlier larger study or a meta-analysis would yield significant clues. For the present, clinicians should screen for glaucoma in hypothyroid subjects only in the presence of additional, well-known risk factors for glaucoma. The association between thyroid-associated orbitopathy and glaucoma is well-known, with several plausible biological mechanisms to explain the link.  For example, in Graves' disease with thyroid associated orbitopathy, the pressure of the enlarged extra-ocular muscles against orbital adhesions might increase IOP. In addition, it is well-known that giving the finite volume of the orbit, any increase in the volume of orbital contents might cause orbital congestion and this orbital congestion might also increase the episcleral venous pressures leading to glaucoma. To support this theory, there is an evidence to suggest that the activity of orbitopathy i.e., the duration of active thyroid-associated orbitopathy is a marker of glaucomatous damage to the disc.  Thus, treatment to reduce the activity of thyroid associated orbitopathy might help in preventing glaucomatous damage and people with long-duration thyroid associated orbitopathy may require special monitoring. Interestingly, hypothyroidism has been associated rarely with thyroid associated orbitopathy, which is classically associated with Graves' hyperthyroidism. 
However, the impact of this hypothyroidism on IOP in the setting of orbitopathy has not been clearly established. The issue of hypothyroidism and glaucoma therefore is more complex than it may seem. Does thyroid hormone therapy in patients with hypothyroidism result in a change of IOP in those without glaucoma? This issue was explored in a recent study that evaluated the effect of levothyroxine therapy on several ocular parameters IOP, central corneal thickness, anterior chamber parameters, retinal nerve fiber layer thickness, retinal thickness and cup to disc ratio.  Thirty three patients were studied with overt hypothyroidism and levothyroxine therapy normalized thyroid hormone levels in all patients; however, there was no change in any of the ocular parameters including intraocular pressures in these patients with thyroid hormone replacement. This carefully carried out study questions the association between high IOPs in hypothyroidism and their resolution with levothyroxine. Clearly, the association between hypothyroidism and glaucoma has not been established and remains an interesting, hypothetical link that deserves further study.
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