This was originally published on attrition.org. I have received a dozen mails over the years from guinea pig owners concerned about the same condition in their piggies. While I am happy to try to help, I really do not have any additional information than what is below. I strongly recommend you talk to a veterinarian that specializes in guinea pigs if possible.
“Hyperthyroidism is a condition in which the thyroid gland is overactive, and makes too much thyroid hormone (called thyroxine and triiodothyronine). Hormones are substances that affect and control many important functions in the body.” 
While many animals have significant differences from humans and each other, many aspects of hyperthyroidism are the same across species. In guinea pigs (Cavia porcellus), this condition is thought to be extremely rare. Recent anecdotal evidence suggests that it may be more common than many veterinarians previously believed. Hyperthyroidism can cause serious problems with other parts of the body such as the heart, kidney and lungs. When diagnosing guinea pigs, problems in these organs are not always thought to be systemic. Given poor breeding habits and the fragile guinea pig body, when a kidney or lungs develop serious issues, there is often little hope for the animal. Doing further investigation to determine why the organs are failing is rarely done for a variety of reasons, primarily the perception of guinea pigs as ‘just rodents’ and the high cost of medical procedures on an animal that is likely near death.
I recently lost a guinea pig, Figlet, to hyperthyroidism, but learned a lot about it during the process. Because it is often difficult to obtain a definitive diagnosis, and because of the lack of information available on the condition as found in guinea pigs, I am publishing my observations and the case history of Figlet. This article is targeted toward guinea pig owners, but will contain a good deal of technical information that will be of interest to veterinarians. I am in a relatively unique position to write this article. First, confirmed diagnosis of hyperthyroidism is very rare in guinea pigs, as was the case for Figlet. Second, I happen to live in the same state as one of the very few doctors who have specifically studied this condition in guinea pigs. Third, I have a strong desire to help educate other guinea pig owners as they often care deeply for their animals, but suffer with the rest of us due to a lack of in-depth knowledge about guinea pig health.
Guinea pigs who develop hyperthyroidism may exhibit several symptoms that help diagnose this condition. These symptoms are not specific to the condition and may manifest to varying degrees or not at all. If you have a guinea pig that exhibits many of these systems, consult your veterinarian. These are described in more detail, as pertained to Figlet later:
- Excessive and frequent urination — Often difficult to observe, especially in a multi-pig cage.
- Enlarged thyroid nodules — Early development may be impossible to detect. Advanced cases may be as large as a pea and easily felt during palpation by owner or vet.
- Excessive energy / very active — May seem to sleep less, run around more, exhibit more energy in or out of cage.
- Increased appetite and thirst for water — May be difficult to observe in a multi-pig cage.
- Weight loss — Even if eating more than usual, an observed steady weight loss.
- Higher temperature and/or heat intolerance — Guinea pig may seek cool surfaces, avoid warm fleece and be warmer to touch than other pigs.
If your guinea pig exhibits these symptoms, consult a veterinarian. Be warned, many vets do not specialize in “exotics” (the term used by vets to describe most rodents, reptiles and birds). They will likely have no experience with hyperthyroidism in guinea pigs and will rely on their knowledge of the condition as pertains to cats. There is only a single test (nuclear scintigraphy) that can definitively diagnose the condition in guinea pigs, and it may be costly or not available from most vets. Blood work can only give an ‘indication’ and palpation of the thyroid gland may not reveal the mass until the condition has progressed. If left untreated, hyperthyroidism can lead to serious problems in the heart, kidney, lungs, spleen and possibly other organs. Surgery to remove the mass is possible, but very expensive and rarely performed.
Diagnosing – Adoption, Known History, Behavior, Symptoms, Testing
Adopted from the Humane Society of the Pike’s Peak Region on June 20, 2008, Figlet likely came from a mediocre life at best and got dumped in a shelter that didn’t care much for animals other than cats or dogs. She was surrendered to the shelter with no prior history. I was down there to look at another “female” pig as listed on their web page, but found two large males instead, both with health problems (after correcting the shelter on the sex of the pigs, they did not update their web page / adoption information). As I was leaving, the employee mentioned another guinea pig that had come in. Originally “Willow” (A762196), the pig was in her own cage, but had no food or hay, and was living on bedding soaked through due to a leaky water bottle (that was empty when I saw her). It was clear she wasn’t going to be in good care, even while in the shelter, let alone with whoever adopted her. Even in a ‘humane society’ or shelter, guinea pigs are often treated with the most minimal of living conditions and comforts. Ten dollars and 80 miles later, Figlet found herself in a cage with hay, pellets, water and daily vegetable platters. Thirty days later, she was integrated with the rest of the pigs without any problem.
Based on her toenail length and condition, she was younger than the shelter estimated. Figlet was energetic and lively from day one, an attribute that would later become a symptom. Since Figlet was an acceptable weight, energetic and displayed no visible symptoms, she was not taken to my guinea pig doctor initially. For the first six months, Figlet gained weight and continued to exhibit normal behavior for a rescue guinea pig. While a bit more energetic than the rest of the pigs, her behavior was not suspect. On December 26, 2008, she weighed in at 808 grams (48 more than adoption weight), which would become her heaviest weight. At that point, she was living with seven other rescue pigs, all 1,000 grams or heavier (a couple over 1,200 grams). The weight gain after adoption was normal, as most guinea pigs (either with a personal owner or in a shelter) are not given a proper diet. Even in retrospect, an initial exam after adoption would have found no issues.
Between December, 2008 and March, 2009, Figlet’s weight started to drop. Her behavior was relatively unchanged, but in an environment with seven other pigs is difficult to properly observe. After dropping to 740 grams, that represented a significant weight loss and I took her in for her initial examination. This began the mystery and problematic diagnosis of her condition. Figlet was taken to Dr. Sharyn Esposito of the Aurora Animal Hospital. Dr. Esposito can be described as the “guinea pig whisperer” for lack of better analogy. She is the most experienced guinea pig vet I have encountered in Denver. Figlet’s first examination found no problems or concerns, and Esposito gave her a clean bill of health on March 14, 2009.
By May, 2009, Figlet continued to lose weight. At this point, I had continuously observed her eating pellets, hay and vegetables, as well as drinking water several times during the day. Her food intake was definitely not an issue (sick guinea pigs often stop eating and drinking). Due to the continued weight loss, I took Figlet back for another examination and additional testing. On June 5, 2009, Esposito did a routine health examination as well as two sets of X-rays that revealed no issues or concerns. Despite the exam and X-ray, both the doctor and I realized something was still not right. During each visit, Figlet’s only other obvious issue was her extremely fast heart beat. Esposito compared the fast heartbeat to a hummingbird, said that she could not reliably count the heartbeats and expressed concern that further testing on the heart would not be possible or reliable.
By August of 2009, Figlet had dropped even further, weighing in at 654 grams. This was over 100 grams lighter than her weight when adopted, and represented a significant loss in weight for a guinea pig. While still energetic and very active, she was still eating a considerable amount, drinking plenty of water and displaying no other signs of a sick guinea pig (lethargy, no food/water intake, anti-social behavior). I took Figlet back to see Dr. Esposito on August 26, 2009 for examination as well as IDEXX urinalysis and IDEXX Fecal/Giardiaag/ova/parasite/direct testing (notes from paperwork). Even after these tests and additional examination, no obvious problems or conditions presented themselves. Dr. Esposito clearly recognized something was wrong with Figlet but could not diagnose the issue. At this point, she suggested additional tests (that were not guaranteed to produce results) or an echocardiogram (although she did not recommend this, as Figlet’s heart beat was significantly faster than most guinea pigs). While I was considering these options, she did additional research. My chart tracking Figlet’s weight follows:
[Note: “pre-veggie” denotes weighing shortly before her nightly vegetable feeding. This weight should be lower than her average weight, as it represents 19 – 23 hours after the last vegetable feeding. “Mid day” denotes weighing approximately 12 hours after the last vegetable feeding. This weight should be somewhat lower than average weights. Water, pellets and some hay were available at all times during these periods.]
Shortly after the latest consult, Dr. Esposito found that Figlet’s symptoms seemed to match cases of hyperthyroidism in guinea pigs. She was very clear that it was not certain, and that research in the condition in guinea pigs was relatively new. She read that research had been done on this condition in guinea pigs by Dr. Matt Johnston of the Colorado State University Veterinary Teaching Hospital in Fort Collins. She suggested I contact Dr. Johnston to discuss Figlet’s case. I called, left a message and spoke with Dr. Johnston on August 29, 2009. After a fifteen minute phone consult, I took the following notes (that I shared with Dr. Esposito), since paraphrased:
- He has seen 5 cases of hyperthyroidism in guinea pigs
- His research on the condition was over, funding ran out
- He believes bloodwork is unreliable, results are not definitive
- In previous cases, surgery fixed it
- Nuclear scintigraphy was the only way to definitively diagnose
- Agreed, Figlet’s behavior and symptoms matched hyperthyroidism
Dr. Johnston asked about specific details of Figlet’s bloodwork that Dr. Esposito took (mainly her ALT) and if her thyroid gland was palpated. Esposito’s exam did not include palpating her thyroid (she palpated every part of her body below her neck, as part of a routine examination) and Figlet’s ALT value was not suspect. Dr. Johnston indicated the next diagnostic steps would be to help in extensive bloodwork for Figlet and ‘healthy’ pigs, to send to the Michigan State Thyroid Lab (which I understood to mean for research purposes), and that he would assist in diagnosis and perform scintigraphy if required. At this point in the conversation, it was not clear if nuclear scintigraphy was the only diagnostic option, if he or Colorado State would perform the test for free as part of their research or if such testing would incur costs for me.
This phone call represented a landmark in Figlet’s diagnosis. An established doctor with significant experience in guinea pig health, a doctor who had specifically done research on a rare condition in guinea pigs and the owner of a questionable guinea pig (me!) all agreed Figlet had a serious condition. Everything pointed to hyperthyroidism. Being the skeptic that I am, I had to wonder what the odds were; I happened to live in the same state as the only doctor that performed research and testing on hyperthyroidism in guinea pigs, he had only seen six confirmed cases and I apparently had a pig with the same condition? If I was a gambling man, I would have won the lottery or hit it big in Vegas. Instead, I had a sick guinea pig that had one long-shot for treatment, maybe, possibly, an hour to the north.
Treatment – Options, Decision, Actions
[Disclaimer: At this point, a certain mental toll was reached. Figlet obviously had serious health problems. I knew she was declining in health and that her options were dwindling quickly. While she seemed happy and healthy, serious weight loss in a guinea pig is a very bad sign. The fact that an established doctor who had extensive experience with guinea pigs could not diagnose the issue was telling. A doctor who specialized in a specific condition said that Figlet’s behavior was suspect but not confirmed, which was disheartening. At this point, my love and affection for Figlet were put on the back burner a slight bit, and a small level of logic and statistics took over. This was unfamiliar territory for me, and my primary concern was Figlet’s health and well being. She was a glorious guinea pig. She was energetic and brought a level of happiness to my life that no other pig had. By this point, I had spent a considerable amount of time and emotional involvement in her case. I could not stand to see her suffer, and that consideration became a primary concern. While she seemed healthy and happy, I knew that guinea pigs were prey animals and would hide discomfort or sickness as long as they could. I realized that she was likely very sick and her health was dwindling quickly.
Transitioning care to Dr. Johnston involved an hour drive each way just to see him. Each visit involved at least three people (as many as five), as Colorado State University at Fort Collins was a teaching hospital. The first visit was for initial diagnostics. Dr. Johnston had heard about Figlet from me on the phone, but had not seen her. Veterinary doctors generally follow the same discipline computer security practitioners follow; “trust but verify”, and I respect that.
Figlet’s first exam with Dr. Johnston revealed the following:
Heart beats ~ 340, normal is 250ish (most vets cannot distinguish this fast of a beat past ~ 300). Dr. Johnston indicated that just stress can take the heart beat to ~ 300, the underlying condition could cause a higher rate. Palpation of the thyroid gland revealed a pea-sized mass. If a doctor specifically looks for this, it is easier to detect via palpation than a cat, as the mass doesn’t sit as far down in the neck area. However, hyperthyroidism in guinea pigs is very rare, so most doctors have no reason to look there. Dr. Johnston acknowledged the hyperactivity, increased appetite and drinking, potential increased urination (large cage with many pigs, difficult to observe), weight loss, fast heart beat and no other specific symptoms of another condition. The provided medical history was informative, but not specific or definitive. Dr. Johnston read her history of urinalysis and fecal tests that proved negative for known conditions or illness. He examined previous X-rays, but only of the body and not the neck region.
The next step in definitive diagnosis is typically blood work and possibly nuclear scintigraphy. With guinea pigs that may have hyperthyroidism, checking the ALT level is indicative but not conclusive. A high value is believed to be a sign, but requires further research. Long term hyperthyroidism can cause serious issues in other parts of the body; the heart beats faster which builds heart muscles and makes heart chambers smaller. This can essentially ‘treat’ problems in the kidney, but can also become an issue unto itself. The condition may also affect the kidney and other organs; the real tricky part is that with hyperthyroidism, the body may begin to treat itself in some areas, especially the kidney. If this is the case, then any bloodwork related to the liver, especially the ALT value, may not be the indicator it could be.
If blood work showed kidney issues, it would really confirm the diagnosis. If no kidney issues are present, then nuclear scintigraphy may be the better option. After the visit, bloodwork and urinalysis were not helpful. Figlet’s liver ALT value was “high normal” but within normal ranges. Further, there were no signs of kidney damage based on the results which suggested her condition may not be as advanced. Her urine was diluted, possibly because of water intake shortly before the test, but had no visible issues.
Based on the evidence at hand, Figlet was his sixth confirmed case of hyperthyroidism in a guinea pig. Between her symptoms, palpation and “everything else”, Dr. Johnston believed hyperthyroidism was a 99% certain diagnosis. Of all the indications, the pea-sized lump on the thyroid gland was the most convincing attribute and lead to him suggesting scintigraphy was not needed. The next choice was determining if surgery to remove the mass was the best option. Colorado State University’s surgical team had only performed the removal of a thyroid mass from a guinea pig two times. While both pigs made it and recovered fully, Figlet was considerably smaller and the procedure is still new and has risks associated with it. If the mass was benign, which Johnston believed it to be, removal meant it would stay gone and lead to Figlet recovering and gaining weight. Forgoing surgery, Figlet would continue to live as she had been, not gaining weight and having her body treat some of the symptoms as it had been.
While this article may downplay the importance of this diagnostic step in Figlet’s case, I do not want to downplay it in the overall diagnostic procedure. Nuclear medicine, involves nuclear imaging or “nuclear scintigraphy” to help diagnose a specific condition in a pet. Some articles on scintigraphy suggest it is a “non-invasive” procedure that does not require surgery, suggesting that it is not harmful to the pet. This may be true for some animals; the process of injecting a small animal with radionuclides and subjecting them to imaging can be extremely stressful and involve a full day procedure. To a human, dog or cat, this may be tolerable. To an animal such as a guinea pig, this may be invasive, stressful and beyond the limits some pet owners are willing to go. Regardless of stress, this is currently considered to be the only way to determine a confirmed diagnosis in guinea pigs.
On the flip side, given it is very stressful to a guinea pig, performing nuclear scintigraphy is not the best option. If every other sign suggests hyperthyroidism, especially palpation of the thyroid gland, then scintigraphy is a costly and stressful procedure that likely isn’t required. If palpation does not result in feeling a growth, scintigraphy is the only method for a definitive diagnosis.
On Thursday, October 15, I lost Figlet. After six months of diagnosis with two doctors and a confirmed case of hyperthyroidism, she did not make it through surgery. After the mass was removed, Figlet’s body didn’t react well. Between her already high blood pressure due to the condition, and the body rejecting the change in her thyroid and several other things, she couldn’t adapt. She passed on the surgery table. She was completely under at the time and did not suffer at all. That doesn’t help me though, as she may have lived out another year or two with the condition before having serious complications with heart and kidney, complications that may have lead to having to choose to let her pass before she started suffering. Instead of that year or more, in trying to do the right thing to make her healthy and ‘normal’, I lost her. In the first weeks after she passed, I felt a lot of guilt over opting for the surgery. I kept thinking that if I let her live out her life, she’d be here today, still happy and hyperactive.
I asked that a necropsy and mass biopsy be performed because I wanted to learn as much as I could about what happened. CSU offered to do both for free (a routine offer), so that they too could study everything and learn from Figlet. Due to miscommunication, I didn’t receive the results until two months later. However, they did set my mind at ease as it showed Figlet was in much worse condition than we had realized. Had I opted not to have surgery performed on her, there is a significant chance she would have started exhibiting more severe symptoms within weeks or a couple months, and that those conditions may have resulted in a poor quality of life or a level of pain that would not have been acceptable. It is unfortunate that she didn’t make it through the surgery, but it was the right thing to do and she did not suffer.
Necropsy – Results, Lessons Learned
The full results of the necropsy are shown below. Because it is highly technical, I spoke with Dr. Johnston one last time to get a layman’s explanation of what happened. These notes are based on our conversation; any errors in conclusions or irregularities are due to my notes and assumptions.
The biopsy confirmed that the thyroid mass was malignant, which they had not seen before. It was a vascular tumor and difficult to remove. When a guinea pig is under anesthesia, it may cause problems to exhibit. In this case, her fast heart beat and high blood pressure were two problems that exhibited. Even if Dr. Johnston had performed an echocardiagram, which I discussed with both doctors, it would not have seen problems in the heart. The edema in the lungs was unexpected and had not come up in previous conversations. Dr. Johnston indicated that it occurred secondary to the problems in her heart. While alive, Figlet’s body compensated for the edema. Under anesthesia though, she lost the compensatory mechanism.
The other significant occurrence was a “thyroid storm” during the procedure. A guinea pig that exhibits hyperthyroidism will have a high level of thyroid hormones (Figlet had 9.6). Several things such as some drugs, pressure on the thyroid gland or stress can cause a spike in the level. What is already a high level (e.g., 10) may jump to 50. This causes the body to be flooded with hormones, essentially causing a shock to the system. Because of the location of the tumor, it required a lot of manipulation to remove. During that part of the procedure, pressure may have been applied that lead to the thyroid storm. This is one of the risks associated with the surgery.
A kidney requires around 75% loss of functionality before test results will change significantly. While Figlet’s kidney did have issues, it wasn’t quite to that point so the blood work was inconclusive. Changes in the spleen were said to be benign and not an issue.
HOSPITAL NECROPSY VTH#: 253917 DL#: 090-24275
Vet/Clinic: Dr. Matt Johnston
Animal ID: Figlet
Date Specimen Taken: 10-16-09
Species: Mam Breed: Guinea Pig Age: 4 Sex: F Died: 10-15-09 History: Losing weight. Been PU/PD and had increased activity. Palpable mass in neck. Further history on file.
1/Left thyroid gland: Thyroid carcinoma, with contralateral thyroid atrophy.
2/Kidney: Glomerulointerstitial nephritis, severe, chronic, with tubular degeneration and necrosis.
3/Lung: Chronic pulmonary edema, moderate.
4/Spleen: Lymphoid hyperplasia, mild.
REMARKS: Gross and histologic findings are consistent with the clinical diagnosis of hyperthyroidism due to thyroid carcinoma. Chronic pulmonary edema may suggest underlying heart disease that could have contributed to anesthetic death.
GROSS NECROPSY: Presented for necropsy is a 4 year old female intact guinea pig in good body condition with minimal autolysis. Submitted with the guinea pig is an approximately 1.5 cm x 0.5 cm surgically removed mass associated with the left thyroid gland. Integument/subcutis: The ventral neck on the left side has been shaved and contains an approximately 1.5 cm sutured surgical incision. Digestive: The liver is diffusely and mildly enlarged with rounded edges, is dark red in color, and is slightly friable. Respiratory: Bilaterally, the lungs are wet and heavy, and exude frothy fluid on cut surface. Urogenital: The right ovary contains multiple cystic structures, up to 1 cm in diameter. The left ovary is within normal limits. The cortex of the right kidney is moderately thinned, with flattening of the cortex at the cranial pole. Endocrine: Bilaterally, the adrenal cortex is mildly thickened, with slight green-brown discoloration of the inner cortex and medulla. The following organ systems are examined and contain no significant gross lesions: Musculoskeletal, cardiovascular, lymphohematopoietic, and nervous.
1/Left thyroid gland: Markedly expanding the thyroid gland and compressing adjacent thyroid tissue is a partially encapsulated multilobulated mass composed of sheets and cords of neoplastic cuboidal cells that occasionally form acinar structures filled with homogenous eosinophilic material (colloid). Neoplastic cells have indistinct cell borders and moderate amounts of amphophilic cytoplasm. Nuclei are round with finely clumped chromatin and variably distinct nucleoli. Mitotic index is approximately 4 mitotic figures per 10 high-powered (400x) fields. Multifocally, lobules of neoplastic cells are present beyond the main capsule of the mass. Focally, there is bone formation within the mass (osseous metaplasia). Right thyroid gland: Multifocally, follicles are collapsed and contain inspissated colloid material. Few remaining active follicles are present (thyroid atrophy). Adrenal glands, 2 sections: Within both sections, the adrenal cortex is moderately thickened, primarily within the zona reticularis. The cortical cells within this region contain moderate to abundant green-brown intracytoplasmic granules.
2/Kidney: Affecting approximately 50% of the renal cortex are multifocal, often linear, aggregates of lymphocytes and plasma cells. There is frequent ectasia of tubules, which are filled with proteinaceous fluid and occasionally cellular casts. Tubular epithelial cells are variably characterized by a combination of the following features: degeneration, necrosis, sloughing into the tubular lumen, regeneration, attenuation, and mineralization. Glomeruli throughout the section are frequently sclerotic, with often marked periglomerular fibrosis. The cortical surface is irregular and undulating. Lymphoplasmacytic inflammation is present within the pelvic submucosa and the peri-renal adipose tissue. Heart: Within normal limits (WNL). Liver: Multifocally scattered throughout the hepatic sinusoids are aggregates of lymphocytes and plasma cells, with occasional extramedullary hematopoiesis. Spleen: Diffusely, the periarteriolar lymphatic sheaths are moderately expanded by mature lymphocytes, with occasional coalescing of hyperplastic lymphoid tissue.
3/Lung: Diffusely throughout the section, perivascular and peribronchiolar spaces are moderately expanded by follicular aggregates of lymphocytes. Diffusely, alveolar spaces contain moderately increased numbers of foamy alveolar macrophages that frequently contain intracytoplasmic brown granular pigment (hemosiderin). Pancreas: WNL. Small Intestine: WNL. Large Intestine, 2 sections: WNL.
Dr. Johnston had been conducting a study on hyperthyroidism in guinea pigs. At some point his funding ran out and he was forced to put it on the back burner. With Figlet’s case (and one other guinea pig), he apparently was able to receive additional funding to perform more testing on thyroid levels in normal guinea pigs. This will lead to a better understanding of test results and knowing when problems are present earlier in the diagnostic phase. Dr. Johnston’s team will be submitting Figlet’s case to the American Veterinary Medical Association for inclusion in the AVMA journal (if accepted).
Figlet was the first guinea pig with carcinoma that underwent this procedure. Her surgery ended up being a valuable learning opportunity for his team and will hopefully go on to help other veterinarians to better understand the condition. It is Dr. Johnston’s belief that hyperthyroidism in guinea pigs is rare, but becoming more prevalent.
This article will be updated with any additional information and/or links to published research about Figlet’s case. If you are a guinea pig owner and have questions about Figlet, her condition, diagnosing or anything else about this article, feel free to contact me (figlet[at]attrition.org) and I will try to help. Remember, I am just a guinea pig owner/rescuer, not a doctor.
I want to give sincere thanks to Dr. Esposito as well as Dr. Johnston and his team. Everyone was very kind and helpful and took great care of Figlet.