Page 1023 - Cote clinical veterinary advisor dogs and cats 4th

504 Hyperthyroidism

• Clinically, the therapeutic goal is to encourage administration. An alternative treatment • If azotemia develops, therapy for CKD
must be used.
should be initiated (p. 169).
weight gain, to improve body condition and ○ A beta-blocker (e.g., atenolol 6.25 mg/CAT • Iatrogenic hypothyroidism must be avoided;
VetBooks.ir and polyphagia, with the overall goal of PO q 12h) can be helpful in some cats diagnosis is based on clinical signs, a T 4 below
haircoat, and to decrease polyuria, polydipsia,
the reference range, and an increased TSH
maximizing quality of life. In cats initially
to nonspecifically control tachypnea,
diagnosed with concomitant azotemia and
hyperthyroidism, achieving T 4 concentrations severe tachycardia, hypertension, and concentration (measured with canine assay).
Hypothyroid cats may have an increased TSH
hyperexcitability.
within reference range may not be possible • Iodine-restricted diet (Hill’s y/d) concentration but a T 4 level in the lower
while avoiding signs of uremia. ○ Iodine restriction should lead to normal- part of the reference range. If persistent
ized T 4 concentrations within 4-8 weeks. hypothyroidism develops, therapy should
Acute General Treatment ○ Patient must strictly eat the diet. Approxi- be initiated (p. 525).
• Hyperthyroidism is a chronic disease, and mately 25% of patients will not eat this
urgent treatment usually is not needed. diet. Recommended Monitoring
• Concurrent congestive heart failure may ○ Clinicians should aim for resolution of • General remarks
require immediate intervention (p. 408). clinical signs and T 4 concentrations within ○ At each follow-up visit, besides history
• The existence of thyroid storm as described in reference range. and physical exam, include systolic blood
humans, characterized by marked hyperther- • Thyroidectomy pressure, body weight, serum biochemistry
mia and decompensation of central nervous ○ Potentially curative profile (especially urea and creatinine), and
system signs, has not been documented in ○ Eliminates grossly abnormal thyroid T 4 and TSH measurement. Elevated TSH
cats. tissue, but ectopic tissue may remain if may be the best biochemical marker of
• However, hyperthyroid cats can get easily radionuclide imaging is bypassed. hypothyroidism in cats.
stressed, and some develop dyspnea/tachypnea ○ Antithyroid medical treatment is essential ○ Aim for T 4 well within reference range.
or open-mouth breathing due to sympathetic for several weeks before surgery to improve ○ After euthyroidism is achieved, reassess q
simulation. Very gentle handling of hyper- the metabolic and cardiac status before 6 months or earlier if adverse effects of
thyroid cats, ensuring as calm and quiet an anesthesia. treatment or signs of hyperthyroidism or
environment as possible, is mandatory. ○ Unilateral thyroidectomy: contralateral hypothyroidism occur.
thyroid gland can eventually become • Medical therapy: periodic CBC, serum
Chronic Treatment hyperplastic or adenomatous (recurrent biochemistry, and T 4 .
See web algorithm Hyperthyroidism: Treatment. hyperthyroidism). • Thyroidectomy: besides the above, postopera-
• Antithyroid drugs and iodine-restricted ○ Bilateral thyroidectomy: may involve tive monitoring of serum calcium if bilateral
diets are reversible options requiring lifelong loss of several or all parathyroid glands. thyroidectomy is important.
therapy. Neither addresses the underlying Postoperative monitoring of serum • 131 I: Effect of treatment on renal function can
disease. calcium is required; if indicated, treat be assessed after 1 month. Hypothyroidism
• Renal function merits special attention. Two as for hypoparathyroidism (p. 519). can occur transiently; unless the cat becomes
very different scenarios are recognized: cats Hypothyroidism is likely. azotemic, the cat should be monitored for
having pre-existing azotemia at diagnosis of ○ Other possible postoperative complications a few months before treating for iatrogenic
hyperthyroidism and cats developing a mild include Horner’s syndrome and laryngeal hypothyroidism. T 4 and TSH normalize in
post-treatment azotemia. The second category paralysis (p. 574). many cats.
of cats does not appear to have a shortened • 131 I therapy
survival time if hypothyroidism is avoided ○ This is considered the best treatment PROGNOSIS & OUTCOME
or treated if it does develop. option for long-term control and cure
• Thioureylene antithyroid drugs suppress of hyperthyroidism in many cats. • Prognosis is excellent for cats. With success-
thyroid hormone production. ○ Treatment renders all hyperfunctional ful 131 I therapy or thyroidectomy, cure is
○ Methimazole 2.5 mg/CAT PO q 12h or thyroid tissue, including ectopic tissue, obtained in most cases.
5 mg/CAT PO q 24h, or carbimazole nonfunctional. ○ In geriatric patients, other conditions
sustained-release tablets 10-15 mg/CAT ○ Disadvantages: special handling facilities may be present or soon develop that
PO q 24h (currently not available in and post-therapy isolation for days to affect survival; median survival time is
North America). weeks are required. The length of isolation approximately 2 years after diagnosis
○ Transdermal methimazole (from a depends on the facility. Before therapy, the with medical therapy and 4 years with
compounding pharmacy) 2.5-5 mg/CAT cat must be eating and able to tolerate the 131 I therapy.
applied inside the ear pinna q 12h has required time in isolation. ○ Cats that are azotemic at the time of
been used in cats that are not amenable to ○ Consult the facility administering the diagnosis of hyperthyroidism have shorter
pill administration or that have sustained radiotherapy to be informed about 1) dis- survival times.
GI side effects with the oral form. Owners continuing medical antithyroid drugs before
must wear gloves. Obtaining remission treatment because duration is controversial PEARLS & CONSIDERATIONS
may take longer (4 weeks). and 2) owner requirements after discharge.
○ Regardless of the formulation used, dosage Comments
is adjusted such that serum T 4 concentra- Possible Complications • Palpation of the ventral neck and measure-
tions are in the lower half of the reference • After therapy for hyperthyroidism, some cats ment of serum T 4 concentrations should be
range; hypothyroidism must be avoided. develop renal azotemia (unmasking of CKD). routinely performed in geriatric cats to allow
○ Common but usually reversible side effects These patients carry a prognosis similar to early detection of hyperthyroidism.
of methimazole and carbimazole include that of cats remaining nonazotemic as long • Several effective therapeutic options are avail-
anorexia, vomiting, and lethargy. as hypothyroidism does not develop or is able. Clinicians should aim for resolution of
○ Pruritus of the head and neck (and treated if it does. clinical signs and serum T 4 well within the
self-induced trauma), hepatotoxicosis, • If T 4 concentration is in the therapeutic reference range.
thrombocytopenia, agranulocytosis, and range for a cat receiving antithyroid therapy, • Monitoring T 4 and TSH concentrations
immune-mediated hemolytic anemia are reducing the dose is not justified if only mild is essential to avoid undertreatment or
uncommon and require cessation of drug azotemia develops. overtreatment.

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