Fluctuation in Serum Thyroid Hormone Concentrations: An Issue in Diagnosis of Feline Hyperthyroidism?

Examining a cat with hyperthyroidism

I am writing this email to you because of your recent article published in the Journal of Feline Medicine and Surgery about diagnostic testing for hyperthyroidism (1). In that paper, you mentioned that hyperthyroid cats can show fluctuation of serum T4 and T3 concentrations (2), leading to difficulty in diagnosis in some cats.

My question is this: When, in your opinion or experience, is the best moment of the day (or the best conditions), to get the blood samples for thyroid hormone analysis to avoid this fluctuation problem?

Thanks! Looking forward to hearing from you.

My Response:

Unfortunately, there is no way to predict if and when this fluctuation in T4 and T3 will occur. So collecting blood samples at a particular time of day would not be helpful.

Remember, however, that this fluctuation is only of clinical importance in cats with very mild hyperthyroidism; in these cats, the serum T4 value may vary from the upper third of the reference range limits to just slightly above normal. In cats with more advanced stages of hyperthyroidism, fluctuation in circulating T4 concentration will occur throughout the day and over a period of a few days, but all T4 values will be high (1-3). Thus, in most hyperthyroid cats, the fluctuation in circulating thyroid hormone levels will not contribute to problems in the diagnosis.

References:

1. Peterson ME. Diagnostic testing for hyperthyroidism in cats: more than just T4. J Fel Med Surg 2013;15:765-777.  
2. Peterson ME, Graves TK, Cavanagh I. Serum thyroid hormone concentrations fluctuate in cats with hyperthyroidism. J Vet Intern Med 1987;1:142-146. 
3. Peterson ME. Feline focus: Diagnostic testing for feline thyroid disease: Hyperthyroidism. Compend Contin Educ Vet 2013;35:E1-E6.

Cystic Thyroid Carcinoma in a Hyperthyroid Cat

Hyperthyroid cat with cystic thyroid tumor

My patient is a difficult hyperthyroid cat with a huge cystic thyroid mass, and I need your advice about how to best treat this cat.  This is a 14-year-old, F/S DSH that has been treated for 2 years with methimazole (5-7.5 mg per day). About 6 months ago, the referring veterinarian noticed that the cat had developed a small cystic thyroid mass in the mid-cervical region, but this cystic tumor has continued to rapidly expand and now extends from the mandible to below the thoracic inlet (Figure 1).

Figure 1: Large Cystic Thyroid Mass

The thyroid mass is easy to palpate, with one large right-sided mass palpable just caudal to the mandible (approximately 2 cm x 1 cm x 1cm) and a second, very extensive irregular mass extending from the larynx to the thoracic inlet (approximately 7 cm x 6 cm x 2 cm) (Figure 1). The cystic thyroid mass is soft and fluctuant in some regions.

Surgical resection was attempted but unsuccessful. Biopsy of the mass identified thyroid adenoma/adenomatous hyperplasia. However, a pulmonary nodule was also identified on a chest film so thyroid carcinoma is still suspected, especially given the large size of the mass.

Since surgery, the cat has continued to lose weight and has now developed a poor appetite. The cystic fluid continues to accumulate within the thyroid mass, and the cat has experienced  2 episodes of severe dyspnea and now has developed mild right-sided Horner's syndrome. The dyspnea was relieved by drainage of some of the cyst fluid. The cystic fluid was somewhat hemorrhagic (Figure 2), and the cat is now slightly anemic.

Figure 2: Cystic fluid removed
from thyroid mass

Routine blood work revealed mild azotemia, dilute urine specific gravity (1.018), and mild non-regenerative anemia (PCV, 23.6%). Cardiac and abdominal ultrasonography was noncontributory. The systolic blood pressure was normal at 140 mmHg.

The cat has been referred to me for radioiodine therapy to treat the cat's large cystic mass and poorly-controlled hyperthyroid state, and to determine whether any other causes could explain or be contributing to her weight loss and poor appetite.

My problem list for this cat includes the following:

• Hyperthyroidism
• Huge cystic thyroid tumor
• Secondary dyspnea and Horner's syndrome due to the compressive effects of the cystic mass
• Chronic kidney disease (IRIS Stage 2)  
• Mild poorly-regenerative anaemia
• Pulmonary nodule (my differentials include a thyroid metastatic lesion, primary pulmonary tumor, or metastatic lesion from an unrelated tumor).

Although the thyroid biopsy was read out as thyroid adenoma, the behavior of this cat's thyroid tumor is more suggestive of carcinoma.  Therefore, I am planning to treat her as a thyroid carcinoma with high-dose (30 mCi) radioiodine next week. I know that I'll have to drain the cystic fluid just prior to her I-131 treatment to minimize the respiratory complications. I plan to maintain her methimazole during treatment given the duration of her disease and size of the tumor.

Do you agree with my treatment protocol? Do these large cystic masses respond to treatment with radioiodine or do they behave differently? What is your experience with this sort of tumor?

This is the largest (hypersecretory) thyroid mass I have seen, and the owners are very realistic about the guarded prognosis and possible complications of treatment. But we really have nowhere else to go other than radioiodine (she was referred to me by a surgeon!).

My Response:

In cats with long-term hyperthyroidism, the development of cystic thyroid masses are not uncommon (1,2). Most of these cysts never become extremely large and the cats remain asymptomatic (i.e., no signs related to the compressive effects of the cystic tumor).

Cystic thyroid nodules have been associated with thyroid carcinoma in dogs, as well as thyroid adenomas or carcinomas in cats (1-5). It is also possible for cats to develop a nonfunctional cystic thyroid adenoma (6-9).

This cat will indeed be a challenge to manage and successfully cure. Here are my thoughts and responses to your questions:

Huge cystic mass extending from her mandible into mediastinum with a solitary pulmonary nodule
Many cats with advanced or chronic hyperthyroidism will develop a large thyroid tumor that can fall through the thoracic inlet into the anterior mediastinal area (10). Similarly, when cats develop thyroid carcinoma, local invasion of tumor tissue into the cranial thoracic area is very common. However, metastasis as a solitary pulmonary nodule would be very rare with thyroid carcinoma.

Therefore, if the pulmonary nodule in this cat is located in the lateral and/or caudal lung lobes, I would say that thyroid metastasis is quite unlikely.

Does the surgical biopsy rule out thyroid carcinoma in this cat?
Confirming thyroid carcinoma in some cats can be difficult. In one study of 8 cats with thyroid carcinoma (5), 2 cases had mixed adenomatous and carcinoma lesions on the same tissue section of their thyroid biopsies. This suggests that carcinoma can arise from a background of benign thyroid neoplasia, at least in some cats.

Therefore, the pathologic finding of thyroid adenoma (adenomatous hyperplasia) can never totally exclude concurrent carcinomatous tissue in a hyperthyroid cat.

The suggestion that thyroid adenoma can transform to thyroid carcinoma makes some sense, since most cats with carcinoma have severe, long-standing hyperthyroidism associated with a large tumor volume (10).  Given that this cat has a huge goiter and has been hyperthyroid for over 2 years, that would make thyroid carcinoma more likely. In any case, given that the thyroid cyst is so large in this cat, very large doses of 131-I would be required to destroy the tumor tissue, even if the cystic thyroid tumor is benign.

Draining the cystic fluid from the thyroid tumor
Of course, you must drain the cystic fluid in order to minimize and relieve the cat's dyspnea. But after you treat with I-131, you need to be prepared to do this in an ongoing fashion during the cats hospitalization.

In other words, the radioiodine will not have immediate effects and the cystic fluid will likely continue to accumulate for at least a few weeks after treatment.

Tumor behavior in hyperthyroid cats with large cystic masses
In this cat, I would expect a fair to good response after radioiodine treatment, with resolution of the hyperthyroidism and a marked decrease in size of the thyroid mass.

However, I have had a number of cats with large cystic masses ultimately require surgical resection to prevent recurrence of cystic fluid following cure of their hyperthyroidism (and return to euthyroid state). In these cats, the cystic fluid continues to accumulate, despite a decrease in the size of the adenomatous mass.

Many of these cystic masses which are deemed "unresectable" before radioiodine treatment will become much easier to remove with surgery after treatment.  Use of radioiodine renders the cyst relatively avascular, thereby making this surgical procedure much more successful.

References:

1. Hofmeister E, Kippenes H, Mealey KL, et al. Functional cystic thyroid adenoma in a cat. J Am Vet Med Assoc 2001;219:190-193. 
2. Phillips DE, Radlinsky MG, Fischer JR, et al. Cystic thyroid and parathyroid lesions in cats. J Am Anim Hosp Assoc 2003;39:349-354. 
3. Wisner ER, Nyland TG. Ultrasonography of the thyroid and parathyroid glands. Vet Clin North Am Small Anim Pract 1998;28:973-991. 
4. Turrel JM, Feldman EC, Nelson RW, et al. Thyroid carcinoma causing hyperthyroidism in cats: 14 cases (1981-1986). J Am Vet Med Assoc 1988;193:359-364. 
5. Hibbert A, Gruffydd-Jones T, Barrett EL, et al. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. J Feline Med Surg 2009;11:116-124. 
6. Liptak JM. Unilateral extracapsular thyroidectomy for a non-functional cystic thyroid adenoma. Aust Vet Practit 1996;26:174-177. 
7. Lynn A, Dockins JM, Kuehn NF, et al. Caudal mediastinal thyroglossal duct cyst in a cat. J Small Anim Pract 2009;50:147-150. 
8. Reed TP, Brisson BA, Schutt LK. Cystic ectopic lingual thyroid tissue in a male cat. J Am Vet Med Assoc 2011;239:981-984. 
9. Nelson LL, Coelho JC, Mietelka K, et al. Pharyngeal pouch and cleft remnants in the dog and cat: a case series and review. J Am Anim Hosp Assoc 2012;48:105-112. 
10.  Peterson ME, Broome MR. Hyperthyroid cats on long-term medical treatment show a progressive increase in the prevalence of large thyroid tumors, intrathoracic thyroid masses, and suspected thyroid carcinoma. Congress Proceedings, 22nd ECVIM-CA Congress (The European College of Veterinary Internal Medicine – Companion Animals) 2012;224.

Radioiodine-Induced Bone Marrow Suppression in Dogs and Cats

In our feline practice, we have have a radioiodine facility and have been treating hyperthyroid cats for a few years. One of my colleagues recently told me that myelosuppression can occur following radiodine treatment. I have never appreciated this in the cats I've treated, nor have I seen it reported. However, it has apparently been reported in dogs treated with very large doses of radioiodine for thyroid carcinoma (1,2).

I'd appreciate your thoughts on this topic. Do you ever see radioiodine-bone marrow suppression in your cats?

My Response:

After administration of radioiodine to a patient (human, dog or cat) with hyperthyroidism, the thyroid gland receives the highest radiation dose because it actively takes up and concentrates the I-131 (3). Other tissues, such as the salivary glands, stomach wall, and bladder also receive a radiation dose higher than that of the total body since these organs will also concentrate iodine. However, the radioiodine uptake in these tissues is much less than that of the thyroid.  The other body organs (such as the bone marrow) will receive a "cross-fire" dose — i.e., radioactivity from photons emitted from the thyroid and, to a much lesser degree, from the salivary glands and stomach  (Fig. 1) (3,4).

Figure 1: Most of the administered dose of I-131 concentrates in the thyroid, but other nearby tissues will receive a much smaller "cross-fire" dose from the gamma radioactivity (photons) emitted from the thyroid tumor.

In general, radiation doses received by the organs that do not normally concentrate iodine, including the bone marrow, are very small; however, the radiation dose delivered to the marrow may be clinically significant if a large enough radioiodine dose was administered to the patient. For example, human patients with Graves' disease or toxic nodular goiter treated with routine doses of radioiodine (e.g., 15 mCi) will not develop myelosuppression (5).

However, patients with thyroid carcinoma are generally treated with very high doses (e.g., 150 mCi to 1000 mCi of 131-I). In this subgroup of patients, severe leukopenia and thrombocytopenia are well-recognized potential adverse effects of high-dose radioiodine administration (6-8).

Dogs with thyroid carcinoma
In dogs with thyroid carcinoma treated with high-doses of radioiodine, bone marrow suppression can develop (1,2,9). Transient hematologic abnormalities (e.g., leukopenia and thrombocytopenia) are most common, but severe and permanent radioiodine-associated myelosuppression has also been reported (1).

Almost all dogs that develop bone marrow suppression have been treated with doses greater than 4 mCi/kg body weight, so it has been recommended not to exceed that dose limit, if possible (1). A higher incidence of bone marrow suppression is also seen in dogs retreated with high-dose radioiodine therapy, so the cumulative 131-I dose may also be an important factor in bone marrow suppression.  In any case, careful and close monitoring is recommended for all dogs treated with high-dose radioiodine.

Cats with hyperthyroidism and thyroid carcinoma
In contrast to dogs, I have never seen a hyperthyroid cat treated with radioiodine develop bone marrow suppression. That's true even in cats with thyroid carcinoma that are treated with 131-I doses as high as 30 to 40 mCi (10-12), which represents a dose range of approximately 5 to 10 mCi/kg on a body weight basis.

For some reason, the feline bone marrow appears to be more resistant to the radiation effects than is the canine marrow. However, this may simply relate to the fact that hyperthyroid cats tend to be much more severely affected than are the dogs with thyroid carcinoma. Therefore, most feline thyroid tumors will take up and concentrate much more of the administered 131-I dose than do canine thyroid tumors (10-15); as more of the administered dose is delivered to the thyroid, this leaves less circulating activity that will be delivered to the rest of the body, including the bone marrow.

References:

1. Turrel JM, McEntee MC, Burke BP, et al. Sodium iodide I-131 treatment of dogs with nonresectable thyroid tumors: 39 cases (1990-2003). J Am Vet Med Assoc 2006;229:542-548. 
2. Adams WH, Walker MA, Danie lGB, et al. Treatment of differentiated thyroid carcinoma in 7 dogs utilizing 131-I. Vet Radiol Ultrasound 1995;36:417-424.
3. Wyszomirska A. Iodine-131 for therapy of thyroid diseases. Physical and biological basis. Nucl Med Rev Cent East Eur 2012;15:120-123. 
4. Lamart S, Bouville A, Simon SL, et al. Comparison of internal dosimetry factors for three classes of adult computational phantoms with emphasis on I-131 in the thyroid. Phys Med Biol 2011;56:7317-7335. 
5. Silberstein EB, Alavi A, Balon HR, et al. The SNMMI practice guideline for therapy of thyroid disease with 131-I 3.0. J Nucl Med 2012;53:1633-1651. 
6. Alexander C, Bader JB, Schaefer A, et al. Intermediate and long-term side effects of high-dose radioiodine therapy for thyroid carcinoma. J Nucl Med 1998;39:1551-1554. 
7. de Keizer B, Hoekstra A, Konijnenberg MW, et al. Bone marrow dosimetry and safety of high 131-I activities given after recombinant human thyroid-stimulating hormone to treat metastatic differentiated thyroid cancer. J Nucl Med 2004;45:1549-1554. 
8. Robbins RJ, Schlumberger MJ. The evolving role of 131-I for the treatment of differentiated thyroid carcinoma. J Nucl Med 2005;46 Suppl 1:28S-37S. 
9. Peterson ME, Kintzer PP, Hurley JR, et al. Radioactive iodine treatment of a functional thyroid carcinoma producing hyperthyroidism in a dog. J Vet Intern Med 1989;3:20-25. 
10. Turrel JM, Feldman EC, Nelson RW, et al. Thyroid carcinoma causing hyperthyroidism in cats: 14 cases (1981-1986). J Am Vet Med Assoc 1988;193:359-364. 
11. Peterson ME, Becker DV. Radioiodine treatment of 524 cats with hyperthyroidism. J Am Vet Med Assoc 1995;207:1422-1428. 
12. Hibbert A, Gruffydd-Jones T, Barrett EL, et al. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. J Feline Med Surg 2009;11:116-124. 
13. Rijnberk A. Hyperthyroidism in the dog and its treatment with radioactive iodide. Tijdschrift voor diergeneeskunde 1966;91:789-794.
14. Rijnberk A. Thyroid tumors and hyperthyroidism in dogs. In: Clinical Endocrinology of Dogs and Cats. Dordrecht/Boston:Kluwer Academic Publishers, 1996;55-59.
15. Mooney CT. Canine hyperthyroidism In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;86-91.

The Many Isotopes of Radioiodine and Their Uses in Medicine

My question is about using radioactive iodine (radioiodine) to treat hyperthyroidism. Is radioiodine-131 (131-I) the only radioisotope we can use to treat this condition? Have you (or anyone else) ever used other radioisotopes of iodine for treatment of hyperthyroidism, such as 123-I, 124-I, or 128-I?

If I-131 is the only form of radioiodine used to treat hyperthyroidism, do the other isotopes of radioiodine have any other medical uses for imaging or therapy?

My Response:

There are 37 known isotopes of iodine (I) from 108-I to 144-I, but only one, 127-I, is stable (i.e., nonradioactive). So as you stated, there are a number of forms of radioiodine other than 131-I (1,2).

Of all of these iodine radioisotopes, only 4 are used as tracers or therapeutic agents in medicine. These are 123-I, 124-I, 125-I, and 131-I. Essentially all industrial production of radioiodine isotopes involves these 4 useful radionuclides.

Of these, however, only I-131 is useful for treating hyperthyroidism. To understand why, let's review these 4 isotopes so you can understand what each of them can do for us in clinical medicine.

Iodine-131
Let's start with iodine-131 (I-131). This radioisotope of iodine has a half-life of 8 days and emits both beta-particles and gamma radiation (see Figure, above) (3-5). The beta-particles, which cause 90% of the tissue damage, are comparatively energetic, travel a maximum of 2 mm in tissue, and have an average path length of 400 μm. Therefore beta-particles are locally destructive to the thyroid tumor but spare adjacent atrophic thyroid tissue, parathyroid glands, and other cervical structures (3-5).

When very high doses are administered, it can also be useful to treat thyroid carcinoma in cats (5-7). In dogs, I-131 is less commonly used, but will work to destroy the thyroid tumor if it is functional and will take up enough I-131 to destroy the tumor (6,8).

I-131 also emits gamma radiation (about 10% of its energy), but this does little actual tissue damage. However, because of this emitted gamma radiation, I-131 can be "seen" by nuclear medicine imaging techniques (i.e., gamma cameras) (2,9). I-131 is almost never used for thyroid scintigraphy, however, since other less-damaging radioisotopes of iodine (I-123, see below) or technetium-99m are preferred in diagnostic situations when only nuclear imaging is required (9).

Figure 1— Decay of radioiodine-131 (I-131)  As I-131 decays (half-life of 8.0 days), it emits both beta-particles and gamma ray energy. The primary emissions of 131-I decay are electron (beta) particles, which are responsible for 90% of the thyroid tumor destruction. Most other radioisotopes of iodine are primarily gamma-emitters (no beta-particle emission).

Iodine-123
Radioiodine-123 is primarily a gamma-emitter with a relatively short half life (13 hours). The main use for I-123 is as a nuclear imaging tracer (i.e., thyroid scintigraphy) to evaluate the anatomic and physiologic function of the thyroid (2,9,10).

I-123 decays by electron capture (EC) to emit a high-speed internal conversion electron (not a beta ray), but this does little cellular damage due to the nuclide's short half-life and the relatively small fraction of such events. In addition, I-123 decays to emit a 159 keV gamma ray, which is well-suited for nuclear imaging (nuclear scintigraphy).

Most authorities consider I-123 to be the radionuclide of choice for imaging the thyroid gland (9,10). However, I-123 is worthless for treating hyperthyroidism because this gamma radiation doesn't help us in destroying thyroid tissue (we need the beta particles emitted by I-131 to do this).

Iodine-125
Iodine-125 (125-I) is a radioisotope of iodine which has uses in biological assays (e.g., radioimmunoassays) and in radiation therapy as brachytherapy (i.e., to treat prostate cancer and brain tumors). It also has a more limited role in nuclear medicine imaging.

Because of its relatively long half-life (59 days) and emission of low-energy photons which can be detected by gamma-counter crystal detectors, I-125 is a preferred isotope for tagging antibodies in radioimmunoassay and other gamma-counting procedures involving proteins outside the body (11). These same properties of the isotope make it useful for brachytherapy and for certain nuclear medicine scanning procedures, in which it is attached to proteins (albumin or fibrinogen), and where a longer half-life than provided by I-123 is required for tests lasting several days.

In addition to its use in biological assays, iodine-125 is also commonly used by radiation oncologists as brachytherapy to treat cancer at sites other than the thyroid, especially in human prostate cancer (12,13). When I-125 is used therapeutically, it is encapsulated in titanium seeds and implanted in the area of the tumor, where it remains. The low energy of the gamma spectrum in this case limits radiation damage to tissues far from the implanted capsule.

Again, I-125 is used therapeutically (to destroy tumor tissue) only in brachytherapy. Because this radioisotope does not emit any beta particles, I-125 is not effective for treatment of hyperthyroidism or ablation of thyroid carcinoma (3).

Iodine-125 has been used for thyroid imaging, but iodine-123 is preferred for this purpose, due to its better radiation penetration and shorter half-life (13 hours vs. 59 days) (10).

Iodine-124
Iodine-124 is a proton-rich isotope of iodine with a half-life of 4.18 days. Its modes of decay are about 75% electron capture and 25% positron emission. Like iodine-123 and I-125, this isotope emits no beta radiation and cannot be used for treatment of hyperthyroidism.

The main use of iodine-124 is to directly image the thyroid using positron emission tomography (PET scanning) (). Iodine-124 can also be used as a PET radiotracer with a usefully longer half-life compared with fluorine-18. In this use, the nuclide is chemically bonded to a pharmaceutical to form a positron-emitting radiopharmaceutical, and injected into the body, where again it is imaged by PET scan (14-16).

References:

1. Johnson PM, Sciarra JJ, Stickley EE. Radioactive iodine isotopes. Rev Argent Endocrinol Metab 1964;10:93-94. 
2. Croft BY, Tsui BMW. Nuclear medicine. In: Medical Imaging: Principles and Practices. Analoui M, Bronzino JD, Peterson DR (eds). Boca Roton, FL: CRC Press, Taylor & Francis Group. 2013:4-1.
3. Wyszomirska A. Iodine-131 for therapy of thyroid diseases. Physical and biological basis. Nucl Med Rev Cent East Eur 2012;15:120-123. 
4. Peterson ME. Radioiodine treatment of hyperthyroidism. Clin Tech Small Anim Pract 2006;21:34-39. 
5. Peterson ME, Broome MR. Radioiodine for feline hyperthyroidism In: Bonagura JD, Twedt DC, eds. Kirk's Current Veterinary Therapy, Volume XV. Philadelphia: Saunders Elsevier, 2013;in press.
6. Feeney DA, Anderson KL. Nuclear imaging and radiation therapy in canine and feline thyroid disease. Vet Clin North Am Small Anim Pract 2007;37:799-821, viii. 
7. Hibbert A, Gruffydd-Jones T, Barrett EL, et al. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. J Feline Med Surg 2009;11:116-124. 
8. Turrel JM, McEntee MC, Burke BP, et al. Sodium iodide I-131 treatment of dogs with nonresectable thyroid tumors: 39 cases (1990-2003). J Am Vet Med Assoc 2006;229:542-548. 
9. Rault E, Vandenberghe S, Van Holen R, et al. Comparison of image quality of different iodine isotopes (I-123, I-124, and I-131). Cancer Biother Radiopharm 2007;22:423-430. 
10. Park HM. 123-I: almost a designer radioiodine for thyroid scanning. J Nucl Med 2002;43:77-78. 
11. Yalow RS. Radioimmunoassay. Annu Rev Biophys Bioeng 1980;9:327-345. 
12. Georgakopoulos J, Zygogianni A, Papadopoulos G, et al. Permanent implantation as brachytherapy technique for prostate carcinoma-review of clinical trials and guidelines. Rev Recent Clin Trials 2012;7:173-180. 
13. Schwarz SB, Thon N, Nikolajek K, et al. Iodine-125 brachytherapy for brain tumours--a review. Radiat Oncol 2012;7:30. 
14. Bailey DL, Townsend DW, Valk PE, et al. Positron Emission Tomography: Basic Sciences. Secaucus, NJ: Springer-Verlag, 2005.
15. Saha GB. Basics of PET Imaging: Physics, Chemistry, and Regulations. Second ed. New York, NY:Springer. 2010.
16. Budinger TF VanBrocklin HF. Positron-emission tomography. In: Medical Imaging: Principles and Practices. Analoui M, Bronzino JD, Peterson DR (eds). Boca Roton, FL:CRC Press, Taylor & Francis Group. 2013;7-1.

Animal Endocrine Clinic: Is a High Serum T4 or Free T4 Level Always Diagnostic for Hyperthyroidism in Cats?

Thyroid scan (scintigraphy) in a cat suspected of hyperthyroidism. Both thyroid lobes are of normal size and shape. Uptake of the radionuclide by the thyroid is also normal, with a thyroid/salivary ratio of 0.8 (normal <1.5). This is a normal study and rules out hyperthyroidism. 
Scintiscan provided by Dr. Michael Broome, Advanced Veterinary Medical Imaging 

Just posted this on my other blog for pet owners but it's relevant information for veterinarians and veterinary technicians as well. To read more, click below:

Animal Endocrine Clinic: Is a High Serum T4 or Free T4 Level Always Diagnostic for Hyperthyroidism in Cats?