Thyroid gland: neoplasia

Introduction

• Hyperthyroidism     is the most commonly diagnosed feline endocrinopathy.
• Hyperthyroidism is associated with hyperplastic or neoplastic changes in the thyroid gland.
• Thyroid tumors:
  • Benign:
    • Adenoma or multinodular adenomatous hyperplasia.
    • The most common cause of hyperthyroidism.
  • Malignant:
    • Adenocarcinoma (2-5% of all tumors).
    • May be associated with excessive hormone secretion (hyperthyroidism) or be functionally silent (non-secretory carcinoma).
• Thyroid tumors can become large if the gland is cystic.
• Thyroid neoplasia is rarely seen in pedigree cats.

Diagnosis

Clinical signs

• Thyroid goiter:
  • Highly palpable on palpation - may be firm or soft, irregular or smooth.
  • Can be very large if cystic.
  • Adenoma or hyperplasia cannot be distinguished from carcinoma on the basis of palpation.
  • May not be detectable if the hyperfunctional thryoid tissue is intrathoracic.
• Thin body condition.
• Poor coat condition.
• Tachycardia.
• Occasionally tachypneic (thyroid storm).
• Resentment of handling.
• Changes associated with complications of hyperthyroidism, eg hypertension induced ocular disease (eg detached retina, hyphema).

Sequelae

Prognosis

• Thyroid neoplasia can generally be successfully managed by one or a combination of the three treatment strategies.
• Medical therapy controls only the systemic effects of elevated thyroid levels, and does not address the underlying tumor. Medical therapy is therefore required life-long following diagnosis of hyperthyroidism. Most cases of benign neoplasia can be managed adequately, however carcinoma cases may be refractory to medical stabilization. Small numbers of cats develop side effects to the treatment including facial excoriation, blood dyscrasia, gastroinestinal disturbances, hepatotoxicity and rarely myasthenia gravis.
• Surgical resection and radioiodine therapy are potentially curative and permanent treatments. Surgery offers a good prognosis provided the cat does not have ectopic tissue and ther are no surgical complications, eg iatrogenic hypoparathyroidism, laryngeal paralysis.
• Radioiodine is associated with the fewest potential side effects. Median survival times of 2 years for benign disease have been reported. Small numbers of cats with carcinoma have been treated solely with high-dose radioactive iodine, however, the survival time appers to be comparable and treatment has been well tolerated.

Expected response to treatment

• Decreased thyroxine levels.

Reasons for treatment failure

• Medical therapy:
  • Inadequate dosing.
  • Inadequate drug absorption.
• Surgical treatment: 
  • Insufficient surgical resection, ie residual tissue due to undetected bilateral cervical lobe involvement or ectopic intrathoracic tissue.
• Radio-iodine:
  • Inadequate dose:
    • Large tumor volume or undiagnosed thyroid carcinoma requiring high-dose radio-iodine.

Sources

Publications

Refereed papers

• Recent references fromPubMed.
• Harvey A M, Hibbert A, Barrett E L, Day M J, Quiggin A V, Brannan R M, Caney S M A (2009)Scintigraphic findings in 120 hyperthyroid cats.J Feline Med Surg11, 96-106PubMed.
• Hibbert A, Gruffyd-Jones TJGJ, Barrett E L, Day M J, Harvey A M (2009)Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment.J Feline Med Surg11, 116-124PubMed.
• Wakeling J, Everard A, Brodbelt D, Elliot J, Syme H (2009)Risk factors for feline hyperthyroidism in the UK.JSAP50, 406-414PubMed.
• Sartor L L, Trepanier L A, Kroll M M, Rodan I, Challoner L (2004)Efficacy and safety of transdermal methimazole in the treatment of cats with hyperthyroidsim.JVIM18, 651-655PubMed.
• Phillips D E, Radlinsky M G, Fischer J R & Biller D S (2003)Cystic thyroid and parathyroid lesions in cats.J Am Anim Hosp Assoc.39(4), 349-354PubMed.
• Wisner E Ret al(1998)Ultrasonography of the thyroid and parathyroid glands.Vet Clin North Am Small Anim Pract28(4), 973-991.
• Peterson M E, Becker D V (1995)Radioiodine treatment in 524 cats with hyperthyroidism.JAVMA207, 1422PubMed.
• Kintzer P Pet al(1991)Thyroid scintigraphy in small animals.Semin Vet Med Surg (Small Anim)6(2), 131-139.
• Peterson M Eet al(1983)Feline hyperthyroidism - pretreatment clinical and laboratory evaluation of 131 cases.JAVMA183(1), 103-110.
• Patnaik A Ket al(1979)Feline anaplastic giant cell adenocarcinoma of the thyroid.Vet Pathol16(6), 687-692.
• Leav Iet al(1976)Adenomas and carcinomas of the canine and feline thyroid.Am J Pathol83(1), 61-122.