Page 823 - Small Animal Internal Medicine, 6th Edition

CHAPTER 48 Disorders of the Thyroid Gland 795

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A

C

B

D

FIG 48.20
(A) Ultrasound image of the right thyroid lobe of a 13-year-old domestic short-haired cat
with hyperthyroidism. A mass is seen in the midregion of the thyroid lobe (solid arrows).
Normal-appearing portion of thyroid lobe (broken arrows). (B) Ultrasound image of the
small (atrophied) normal left thyroid lobe (solid arrows). Left thyroid lobe (small arrows).
Results of the ultrasound examination supported unilateral disease affecting the right
thyroid lobe, which was confirmed with a sodium pertechnetate scan. Longitudinal (C)
and transverse (D) ultrasound images of a markedly enlarged left thyroid lobe (measuring
approximately 1.1 × 2.2 cm) in a 14-year-old domestic short-haired cat with
hyperthyroidism. The right thyroid lobe was similar in size. Results of the ultrasound
examination support bilateral disease with symmetric enlargement of both thyroid lobes,
which was confirmed with a sodium pertechnetate scan.

incidence of adverse reactions associated with its use is lower 2.5 mg per day (i.e., 5 mg in the morning and 2.5 mg in the
than that associated with the use of propylthiouracil (Table evening) twice daily, and the same parameters are evaluated
48.7). Adverse reactions are less likely to occur when the 2 weeks later. The dosage should continue to be increased
dosage of methimazole is started low and gradually increased every 2 weeks by 2.5-mg/day increments until the serum T 4
to effect. The recommended initial dose of methimazole is concentration is in the lower half of the reference range
2.5 mg administered orally twice a day for 2 weeks. If adverse without creating hypothyroidism or until adverse reactions
reactions are not observed by the client, if the physical exam- develop. Timing of blood sampling after oral methimazole
ination reveals no new problems, if results of a CBC and administration does not appear to be a significant factor in
platelet count are within reference limits, if the serum creati- assessing response to methimazole treatment. Serum T 4 con-
nine concentration has not increased, and if serum T 4 con- centrations decline into the reference range within 2 weeks
centration is in the upper half of the reference range or once the cat is receiving an effective dose of methimazole;
higher after 2 weeks of therapy, the dose is increased by clinical improvement is usually noted by clients within 2 to