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In people, the thyroid gland is located ventral to the trachea with both lobes connected by an isthmus. In cats, there is no isthmus, and thus, both lobes are separated and located on both sides of the trachea. It is made of glandular tissue with cells in follicles. Follicles are filled with colloid that incorporates the main storage of thyroid hormones. There are also parafollicular cells outside of the follicles that secrete calcitonin. The synthesis of thyroid hormones occurs within the thyroid follicular cells and the colloid. Iodine that is taken into the body is converted to iodide in the gastrointestinal tract. It is transported into the follicle cells by the Na+/I- symporter, is then oxidized in the follicular cell to the active form, and then moves into the colloid. A molecule called thyroglobulin is synthesized and undergoes exocytosis into the colloid. The thyroglobulin is then iodinized by thyroid peroxidase into monoiodotyrosine (MIT) and diiodotyrosine (DIT). MIT and DIT are either linked together to form T3, or DIT links with another DIT to form T4. Thyroglobulin then undergoes endocytosis into the follicular cells, then fuses with a lysosome that cleaves off T3 and T4 via hydrolysis. T3 and T4 then leaves the thyroid follicular cells to peripheral tissues. In those tissues, T4 is converted to the active form of T3. Thyroid hormone penetrates the cell membranes and acts on the nucleus to initiate transcription of mRNA. This leads to increases in consumption of oxygen, increases in heat production, enhancement of the sympathetic nervous system effects by beta-adrenergic receptors, increases in heart rate and contractility, and increased lipolysis, among a number of other actions. Thyroid hormone is essential for normal growth. Without it during development, animals will be born a disproportionate dwarf or will die in utero.
Feline hyperthyroidism (FHT) is the most common endocrine disease in the cat. It has a 6% incidence rate in all cats over the age of nine years old and that incidence rate is increasing. There is a thought that, potentially, this has to do with an increase in goitrogens in commercial cat food with varying degree of iodine as well. Hyperthyroid cats will most likely have bilateral adenomas with a greater than 50% chance that they will be asymmetrical. Unilateral disease occurs in much fewer patients. Cats can also have ectopic thyroid tissue anywhere from the area of the thyroid gland to the base of the heart. Thyroid carcinoma in cats is very rare. Signs of FHT commonly include weight loss, goiter, a thin Body Condition Score, and heart murmur. Common diagnostics to detect FHT are total T4 and free T4. Less common diagnostics include T3 suppression test, TRH stimulation test, and thyroid scintigraphy. Treatments for FHT include antithyroid drugs, limited iodine foods, radioactive iodine, and thyroidectomy, with each of them having their own set of advantages and disadvantages.
In the body, thyroid hormone and thyroglobulin are the only molecules that require iodine. Therefore, all ingested iodine goes only to the thyroid gland through the Na+/I- symporter. If iodine is tagged with radioactivity (I-131), beta particles will destroy the local tissues in the hyperactive adenomas. Adjacent normal tissues can also be destroyed by collateral damage. Radioiodine can be administered subcutaneously, intramuscularly, or per os. I-131 is indicated for cats who have methimazole reactions, owners who can’t medicate their cats, and owners who are looking for a cure. Additionally, it is considered for cats who don’t like the limited iodine diets and owners who wants to avoid thyroidectomy. Diagnostics that are needed before therapy include full labs with three months of presentation (ideally, a set of labs on and off of methimazole), imagining of the chest and abdomen, and thyroid scintigraphy. As much as possible can be done at the local vet. After the appropriate diagnostics are performed, a dose of I-131 is carefully formed based on the results and patient condition. Some other facilities that perform radioiodine therapy may give the same dose to each patient (usually 2 mCi). Angell uses 2–3 mCi as a starting point, and increases or decreases may be made based on results of the diagnostics. After treatment, we house the cats in a particular ward, where we monitor them for any side effects of the treatment (anorexia, laryngitis, dehydration, etc.) and give any appropriate treatments. We keep the cats for a minimum of 72 hours post treatment, per Massachusetts state law. It is at the owner’s discretion if they want to take the cat home on all restrictions (contact and litter) or to board them for the full restriction time table of 17 days after treatment, when contact restrictions are lifted. Litter restriction can be lifted after 10–14 days. A T4 and check of kidney values should be rechecked in one month and three months post administration. If the patient is hypothyroid at the three-month mark, then they will likely need supplementation with L-thyroxine. If the patient is persistently hyperthyroid, they can pursue any of the previous treatment options illustrated, but most commonly, they are retreated with radioiodine and will be euthyroid post-procedure in most cases.
Treatment of these particular cats is challenging with methimazole and often results in many rechecks with many dosing adjustments. Cats usually are on high doses of methimazole when they come in to the clinic with unpredictable T4 values. I-131 is recommended at this point with the goal either to make them hypothyroid so that we can supplement with L-thyroxine for a more predictable response to treatment or to attempt to bring their T4 in to a more manageable range with the intent of methimazole having better control when administered. For cats who have thyroid carcinoma, we give a dose of 30 mCi to ablate all thyroid tissue, rendering the patient hypothyroid. Thyroid scintigraphy helps us guide whether we think that it is a carcinoma versus a very active adenoma. Carcinoma cats usually have an ill defined and locally invasive amount of thyroid tissue on their scan, whereas an adenoma will have smooth margins and a very defined capsule. Carcinoma cats are always supplemented with L-Thyroxine. High thyroid levels with a scan suggestive more of an adenoma will get anywhere between 5-10 mCi. Because of the higher dose than other cats, we warn owners of the increased risk of becoming hypothyroid, but it is not our actual goal like with thyroid carcinoma cats. Owners are then prepared if the cat needs to go on L-thyroxine or not. Because of the increased doses given to both of these types of patients, the owners need to be prepared for longer lengths of stays, increased amount of post-radiation side effects (nausea, vomiting, diarrhea, inappetence), and increased cost of treatment if supportive care measures are implemented. Rarely, do these cats have to be moved to the general hospital for supportive cares measures. Every precaution is taken to protect staff from the radiation levels, including lead blocks to prevent exposure. These cats are usually in the hospital for an additional 2–4 weeks than the standard dosing.
