Melissa Kaplan's
Herp Care Collection
Last updated January 1, 2014

Hypothyroidism in Green Iguanas

©2000 Melissa Kaplan

What is the thyroid gland?
The hormones produced by the thyroid gland aid in skin production and shedding, regulating the metabolism, the energy and activity levels, growth, and temperament. In other words, the thyroid is closely bound with many facets of the organism's health and well being. I say "organism" because all animals that have a thyroid gland are all affected similarly when it is working...and when it is not. To help it function properly, the thyroid of all animals needs sufficient amounts of iodine on a regular basis. To help ensure humans get enough iodine, regular salt (table salt) has been "enriched" with iodine, that is, iodine is added to it to help those who may not otherwise be getting enough iodine in their regular diet. Fortunately, feeding a properly constructed plant diet will provide all the iodine an iguana needs - you never have to add iodized salt to their food!

What is hypothyroidism?
Unfortunately, some plants contain goitrogens - compounds that bind the iodine in food, preventing the body from utilizing it. This is easily dealt with by simply not feeding your iguana a lot of those vegetables: broccoli, cabbage, cauliflower, bok choy, Brussels sprouts, kale, soy (tofu), and grains. Some of the chemicals found naturally occurring in plants that are known to bind iodine include:

• anthocyanin
• benzyl-thiocyanate (a possible goitrogen)
• goitrin
• iodine
• phloroglucinol
• vitexin

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.

What causes it?
Goitrogenic foods are those which contain iodine binders. When the body cannot get enough iodine, a goiter forms, which impairs thyroid function, causing hypothyroidism. Feeding goitrogenic foods as a significant part of the diet, human and iguana, can thus lead to hypothyroidism. This leads to reduced activity levels, reduced metabolic rate, muscle and joints aches and pains, and we know in humans (and, from personal experience, dogs) it also causes moderate to severe headaches, vision problems, and hair loss, as well as a host of other systems.

Of the plant parts most commonly fed, the plants with problematic levels of one or more goitrogens:

• Cabbages (Red, white, etc.) (Brassica oleracea var. capitata l. var. capitata L.)
• Garden cress (Lepidium sativum)
• Grapes (Vitis vinifera)
• Kale (curly kale, Kitchen kale, Scotch kale) (Brassica oleracea var. sabellica l. var. acephala DC)
• Mango (Mangifera indica)
• Pea (Pisum sativum)

Some plants have both iodine and iodine-binders, such as:

• Kelp (Fucus vesiculosus)
• Soybean (Glycine max)

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.

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.

Soy, and food products made from soy (tofu, tempeh, soy nuggets, soy milk, soy yogurt, soy cheese, edemame, etc.) contains fat (impedes calcium uptake), goitrogens (impedes iodine uptake), and oxalates--as much in a 1/2 cup of tofu as in 1/2 cup of spinach (over 200 mg). (Source: OHF, Oxalate Content of Food)

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:

Iguana Care, Feeding and Socialization, by Melissa Kaplan
There is a nonfat tofu cake available that would be great for use except for the other problem associated with tofu. Tofu contains goitrogens, phytocompounds which interfere with the thyroid gland's uptake of iodine from the diet. Over time (and it may be a period of several months, depending on how much other highly goitrogenic foods are being fed), the thyroid gland will malfunction, causing hypothyroidism.

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.

Notes on Dietary Constituents for Herbivorous Terrestrial Chelonians and Their Effect on Growth and Development, by Andy Highfield:
Dietary Iodine: Fibrous goitre or hypothyroidism is commonplace among captive herbivorous chelonians, particularly Chelonoidis elephantopus and Megalochelys gigantea maintained in zoological collections and which have been fed diets rich in vegetables containing high levels of anionic goitrogens (e.g. glucosinolates and thiocyanates) such as cabbage and kale. The condition has also been observed in hatchling T. graeca and T. marginata subjected to an identical dietary regime. The consumption of the responsible class of vegetables needs to be limited and in addition a multi-mineral supplement containing traces of iodine at a suggested dose rate of 6-10mg per Kg of supplement should by routinely provided with every meal. Vegetation from mountainous areas, or from soil rich in limestone may be lower than average in dietary iodine.

Hypothyroidism in Humans
Hypothyroid disease is one of the top five underdiagnosed and mis-diagnosed diseases. It is especially prevalent in women. It is mis-diagnosed or underdiagnosed because the common tests used to test thyroid function aren't testing the right things, something most internists and family practitioners are not aware of. For more information on thyroid disease in general, and hypothyroidism in particular, visit About.com's excellent Thyroid forum and Mary Shomon's Thyroid Information site. A good place to start is with her Basic Information on Hypothyroidism, Hyperthyroidism, Nodules, Cancer article.