We’ll start this one off by saying that when we’re talking about human biology, hormones affect just about everything. As anyone who’s ever been through puberty can attest, hormonal changes can completely alter a human body. Changes in sight is, of course, no different.
Hormones and hormone ratios are crucial parts of the body’s system for maintaining a healthy visual system. In this article, we’ll go into a little more detail about what hormones are, and what exactly we mean when we talk about their effects on vision.
To use a broad definition, hormones are chemicals that trigger various processes in an organism. They’re produced and distributed by the endocrine system, a body-wide switchboard of glands that create these tiny biological messengers. There’s an enormous variety of known hormones, each with their own functions, and each in some way integral to the proper functioning of a human being.
Testosterone and estrogen, the sex hormones, are two obvious examples. The sheer effect that these two chemicals can have on a human is well illustrated by the changes brought on by puberty. Estrogen, for instance, affects everything from the formation of bone to the function of the lungs. The ways in which hormones function – and the ways in which they sometimes fail to function – is a field known as endocrinology.
While it’s fast expanding, and has has seen enormous strides of late, endocrinology is in some regards still in its infancy. The exact role of hormones in many parts of the body is only just beginning to be fully understood; however, several recent developments have helped shed some light on how hormones can affect our eyes.
Thyroid Hormones and Eye Pigment
While we’ve known for some time that thyroid hormones were involved in the development of eye pigmentation, it was only recently that researchers began to discover the extent of that relationship. To understand the role of thyroid hormone though, we’ll have to first quickly explore the human eye. The ability of the eye to perceive light starts with photoreceptors, small cells that activate in response to light.
Human eyes contain two varieties: rods and cones. Rods are responsible for low light and peripheral vision, and are largely concentrated around the edges of the retina. One thing they’re not so great at is perceiving color and fine detail, which is where cones come in. Cones are mostly found in the fovea, an area on the center of the retina that gives us acute sight in the middle of our visual range.
Cones owe their ability to distinguish color in part to thyroid hormones. The pigment, or coloration of a cone dictates what wavelengths of light the cell is sensitive to. Thyroid hormones can trip a reaction in a cone cell that prompts it to begin manufacturing a given variety of pigment – in essence determining what type of light it can respond to.
For many years, researchers assumed that this process took place largely during the earliest stages of development, but a European team of researchers from Frankfurt and Vienna found reason to believe that thyroid hormones play a role in pigmentation over the entire course of an organism’s life.
Studies in mice found that the hormone/cell exchange still took place for weeks after a mouse’s birth. The findings suggest that cone cells, far from being fixed to one color for life, can change dynamically over time.
Estrogen and Dry Eye
While dry eye is relatively common in any population, postmenopausal women may have some of the highest odds of encountering the problem. Menopause is a gradual process that occurs as a woman stops undergoing the menstrual cycle. Sex hormones, including estrogen, taper off during this period, kicking off a plethora of physical changes. Classic signs include hot flashes, back pain, and mood swings – all of which are brought on by dropping hormone levels.
Dry eye is also on that list. Over seven percent of women 50 years or older will experience dry eye, a potentially serious condition that can result in infection and even permanently impaired vision. The cause seems to be the disruption of chemical signals that help maintain a healthy level of tears in the eye, though the inflammation that can accompany menopause may also play a role.
Hormone replacement therapy has not been shown to have any benefit in treating dry eye, but conventional therapies, such as lubricating eye drops and ointments, can still provide a measure of relief.
Pregnancy and Vision
The cascade of hormones brought on by pregnancy can also have some major effects on vision. Most are harmless and short-lived, and disappear once a pregnancy is brought to term. However, it’s not unusual to encounter dry eyes or, strangely enough, blurry vision caused by fluid retention in the eye. Two notable exceptions to the “safe and temporary” rule are the effects of preeclampsia and gestational diabetes.
Preeclampsia is a potentially serious disorder characterized by high blood pressure and changes in urine composition. It can also manifest as temporary loss of vision, or strange perceptions of light, such as photosensitivity or auras.
Gestational diabetes is a variant of diabetes that appears with pregnancy. If it does develop, a mother’s eyes may be at risk. The high blood sugar and blood pressure associated with diabetes can damage tiny blood vessels near the retina, which can potentially cause permanent damage to the affected eye.