- This case report earned Honorable Mention in the 2018 AEMV Lafeber Company Student Case Report Contest.
- Two guinea pigs suspected of having hyperthyroidism based on weight loss, presence of a cervical mass, and elevated total thyroxine levels were confirmed to have active thyroid masses via scintigraphy with Technetium-99 and treated with radioactive iodine therapy (I131).
- Both patients responded initially with weight gain and a decrease in the total thyroxine.
- This case series describes the use of scintigraphy to aid in confirming a diagnosis of hyperthyroidism, as well as to show the benefits and effectiveness of using radioactive iodine therapy (I¹³¹) to treat hyperthyroidism in guinea pigs in order to provide the best treatment protocol.
History and physical examination findings
Two cases were referred the University of Tennessee Veterinary Medical Center for further diagnostic workup and treatment of suspected hyperthyroidism.
- Case 1: A 3-year-old castrated male Abyssinian guinea pig (Cavia porcellus) was referred for a cervical mass.
- Case 2: A 3-year-old spayed female white crested guinea pig was referred for radiation therapy for suspected hyperthyroidism.
In each case the presented clinical signs included weight loss and a palpable mass on the ventral neck with no other abnormalities noted on physical examination.
For Case 1, an ultrasound along with a fine needle aspirate (FNA) of the cervical mass was performed at initial presentation. On ultrasound, the mass appeared cystic with mineralized clustering of granules (Fig 1). The FNA showed serosanguineous fluid which was interpreted as chronic and active hemorrhage within the cyst. The patient returned 2 days later for a complete blood count (CBC), plasma biochemistry and TT4, which were within normal limits except for an elevated TT4 of 5.43 μg/dL (reference range 2.3-3.5 μg/dL).6,7,8 A thyroid scintigraphy scan was performed 17 days later with Technetium-99 which showed increased radioisotope uptake of the right thyroid gland, supportive of a functional right thyroid gland adenoma or carcinoma (Fig 2).
For Case 2, a thyroid scintigraphy scan was performed along with a CBC, serum biochemistry panel and TT4. The scintigraphy scan showed increased radioisotope (Technetium-99) uptake of the left thyroid gland with a smaller and less intense uptake in the right thyroid gland (Fig 3). Total T4 was elevated at 6.6 μg/dL (reference range 1.1-5.0 μg/dL).7
Treatment and outcome
Surgical resection was initially considered the course of treatment for Case 1. However, due to continued weight loss that patient was no longer recommended for surgery. In both cases radioactive iodine (I131) treatment was chosen as the preferred course of treatment. Case 1 was injected with 1.3 mCi of I131 subcutaneously, while Case 2 was injected with 1 mCi of I131 subcutaneously. For both cases the patients were then held in radioactive isolation and monitored daily for seven days following injection.
Clinical signs in Case 1 improved, within three weeks the cervical mass had shrunk from 2cm in diameter to 1.5×1.7cm in size and the patient regained weight from 810g to 865g. At the one month follow up appointment the TT4 level had decreased to 1.61 μg/dL which was considered normal.7 However, about three months after radiation therapy the patient started to gradually lose weight again (770g) and the TT4 started to increase (1.94 μg/dL). At 7 months post radiation therapy the patient presented with a urinary tract infection, he had lost weight (651g) and the TT4 had increased to 3.55 μg/dL. Forty-one days later the patient died at 543g. The cause of death was severe cystitis with bilateral ureteral obstruction. The right thyroid gland was 25x15x15 mm and had a follicular cystadenoma with stromal osseous metaplasia.
Clinical signs for Case 2 continue to improve at 4 months following radiation therapy by gaining weight and a decrease of TT4 levels into the normal range when measured one-month post therapy (3.2 μg/dL).
Hyperthyroidism, along with other endocrine neoplasms, seem to be increasing in frequency over the last decade.5 In a recent study done at Northwest Zoopath, out of 236 neoplasms found in guinea pigs 19 (3.6%) of them were thyroid neoplasms.2 There seems to be a slight predilection for females.6 It is important to note that not all thyroid tumors are functional. There have been some reports of non-functional tumors as well, and the distribution of adenomas and carcinomas is currently unknown.3
Overall, hyperthyroidism can occur in a guinea pig at any age. However, most reports occur in guinea pigs older than 3 years of age.8 Both of the cases described here were 3 years old. The typical clinical signs of hyperthyroidism in guinea pigs include, hyperesthesia, nervousness, hyperactivity and weight loss despite an increased appetite as well as a palpable thyroid nodule in some cases.8 Both guinea pigs in this report showed signs of weight loss despite an increased appetite, as well as a palpable thyroid mass.
Hyperthyroidism can be diagnosed through various methods, including measuring total thyroxine (TT4) levels, ultrasound, radiographs, CT, MRI and nuclear scintigraphy. In guinea pigs TT4 and triiodothyronine (T3) can be unreliable for both diagnosis and treatment monitoring so it is important to use a multimodal approach.6 Euthyroid sick syndrome can cause a hyperthyroid individual to have TT4 levels within the normal range. Obtaining a free T4 may be more beneficial because it is less likely to be affected by concurrent illness.8 An FNA of the mass, with or without ultrasound guidance, may help to differentiate between adenoma and adenocarcinoma, although hemorrhage is a risk.4
Many of the same treatment options for cats apply towards guinea pigs with hyperthyroidism. Those options include, medical treatment with methimazole or carbimazole, surgical excision, percutaneous ethanol injection and radioactive iodine treatment.6 The percutaneous ethanol injection was only mentioned in one study, with poor prognosis so it is not recommended by most. Medical treatment with antithyroid drugs such as methimazole and carbimazole has shown to be effective at controlling the clinical signs of hyperthyroidism, however, there have been no studies to determine the optimal dose and dosing regimen for guinea pigs as dosing has been extrapolated from cat studies.1 One downfall to medical management is that it requires daily, life-long treatments which might not be feasible for some clients.
A more permanent treatment option is a thyroidectomy, but due to the anatomy it is very common to cause recurrent laryngeal nerve damage or have accidental removal of the parathyroid gland leading to parathyroid insufficiency.6 Due to these complications, other more permanent treatment options have been pursued such as radioiodine (I131) treatment as used in these two cases. This treatment may be one of the best options because it can result in long-term control of the disease and potentially be completely curative, and it does not affect the parathyroid tissue.1 Another advantage to radioiodine treatment is that is has the ability to treat ectopic tissue as well.6
Both Case 1 and Case 2 responded initially to their I131 treatment as expected with decreased TT4 levels and weight gain. It is unclear why Case 1 later had recurrence of the thyroid tumor. Further research is needed to determine the median survival time following I131 treatment in guinea pigs.
Treatment of hyperthyroidism in guinea pigs with radioiodine (I131) causes a decrease in the TT4 concentration in the blood as well as resolution of clinical signs such as weight loss based on the information collected during these two cases. In conclusion, radiation therapy should be offered as a treatment option for guinea pigs with hyperthyroidism confirmed by nuclear scintigraphy.