Hypothyroidism in Green Iguanas
©2000 Melissa Kaplan
What is the thyroid
What is hypothyroidism?
Some plants may have just one goitrogen, while other plants may have more than one. Some have very small amounts, while others have higher amounts. If you feed your iguana plants that are high in goitrogens, and feed them regularly and in quantity, your iguana's thyroid gland will not be able to get the iodine it needs and so will start to suffer a decline, not putting out the various hormones and chemicals your body needs. This is called hypo- (low) thyroid, or hypothyroidism.
Hypothyroidism in iguanas leads to iguanas who are sluggish, lethargic, chubby but slow growing, and generally very sweet natured without having had to work much at being tamed. Keepers of such iguanas are amazed at the difference within a couple of weeks after they eliminate the large quantities of goitrogenic foods from their iguana's diet.
Hyperthyroid, a medical condition in which the thyroid becomes overactive, is uncommon in iguanas. Symptoms may include overly rapid growth, thickened bones, and a nervous, jittery temperament. (The latter symptoms should not be confused with the normal behavior of an untamed or otherwise high-strung iguana.)
You can look up goitrogens in plants at the Phytochemical and Ethnobotanical Databases site. Keep in mind, please, that not every plant is in the database, only ones that the US Agricultural Research Service has already analyzed and entered the data into the databases. Thus, some common food plants may be missing, as will plants commonly fed to food and other domestic animals.
Of the plant parts most commonly fed, the plants with problematic levels of one or more goitrogens:
Some plants have both iodine and iodine-binders, such as:
Like spinach, which has both calcium and calcium oxalate, the goitrogen in kelp, for example, may be enough to bind the iodine it contains and other iodine in the diet.
It is also important to note that I have not been able to locate data on all plants. It is known that broccoli and cauliflower can affect thyroid hormone levels so they do have goitrogens in them. The source I use most extensively, the Duke University Agricultural Research Service's Phytochemical & Ethnobotanical Database, does not have information on many of the Brassica.
As when dealing with food choices and oxalates, it is important to weigh the overall importance of a plant food against the overall diet. Diets that contain staples which include regular and significant quantities of broccoli and kale, for example, are going to provide sufficient amounts of goitrogenic chemicals to cause hypothyroidism. Goitrogenic foods can be fed, but do so in moderation, and use foods lacking or containing very low levels of goitrogens as dietary staples, leaving the others for occasional treats.
What hypothyroidism and goitrogens are not.
Hypothyroidism and goitrogens have nothing to do with calcium deficiency (metabolic bone disease), the mineralization of soft tissues (due to high levels of calcium oxalates or oxalic acid intake), thiamine deficiency, or kidney failure.
Calcium oxalate binds calcium, not iodine. When the body does not have sufficient calcium to both maintain bone density (and to grow bones in young animals, or develop eggs properly in gravid females), bones become less dense - like a sponge grown more holey. Since calcium is used throughout the body, by the cardiopulmonary system, gastrointestinal tract, the brain, and other organs and systems, hypocalcemia (calcium deficiency) can be very serious if not diagnosed and treated promptly, and the underlying conditions which caused it, corrected.
In the case of gravid females, if a female is tested and shows serum calcium within the normal range, she is in fact hypocalcemic - she will not have enough calcium to support her own body's requirements as well as those of her developing embryos and will not be able to make shells for them. The serum levels are "normal" because the calcium is being mobilized from her bones to service her systemic needs. Once the bones have been stripped, she will die. In a healthy animal whose Ca:P:D3 is in good shape, the calcium normally lifted from the bone but is then redeposited into the bone so the bone doesn't get porous. In a situation where there is insufficient calcium, and so there isn't enough for the bones and the rest of the body, it is usually the bones that are affected first - which is why most people associate MBD with swollen long bones or jaws...when in fact reduced appetite and constipation are also signs of severe to advanced MBD. A healthy gravid female who has sufficient calcium for her body's needs and those of her developing eggs will test out as being slightly hypercalcemic. In the case of a gravid female, this is good and desirable.
Kidney function is not directly affected by goitrogenic foods or metabolic bone disease. They can be affected by high calcium oxalate intake as oxalates will causes crystals in the kidneys just as easily as it mineralizes the heart, lungs, liver, muscles and joints, but it does not target the kidneys. As more of the healthy organ and muscle tissue is replaced or interspersed with these crystals, the organ function starts to decline - in advanced cases, they may cease functioning altogether or be impaired to such an extent that they can no longer contribute to homeostasis (health) and so the animal falls ill. If the organ function cannot be restored, or the degeneration halted when there is still enough left to support sufficient functioning, the animal dies.
Kidneys are also affected by the body's fluid volume. Low volume = dehydration = impaired kidney function. If sustained over the long term, it causes kidney failure. This problem can be compounded in iguanas and other reptiles through the use of antibiotics, many of which are already hard on the kidneys. Without enough fluids in the body to keep the kidneys well flushed, kidney damage will occur...leading to a potential for kidney failure. Fluid volume has nothing to do with oxalates nor with goitrogens and hypothyroidism.
References to hypothyroidism at my site:
Care, Feeding and Socialization, by Melissa Kaplan
When fed in excess, vegetables including broccoli, Brussels sprouts, kale, cabbage, cauliflower, and bok choy cause hypothyroidism, a thyroid deficiency. This slows down the metabolism, causes lethargy, and muscle and joint aches. The animal gains weight due to the slowed metabolism and lowered activity levels, but overall growth is abnormally slowed. These vegetables may be fed in small amounts in addition to the regular vegetables, mixed into their salad, but should never become the primary ingredients in the salad.
on Dietary Constituents for Herbivorous Terrestrial Chelonians and Their
Effect on Growth and Development, by Andy Highfield:
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