Thyroid gland

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Precontrast computed tomography transverse image of the normal thyroid gland (arrows) at the level of the 3rd cervical vertebra in a dog[1]

The normal canine thyroid gland consists of a right lobe, which extends from the caudal border of the cricoid cartilage of the larynx to the level of the 5th tracheal ring, and a slightly more caudal left lobe, which extends from the level of the 3rd to that of the 8th tracheal ring[2].

Infrequently, an isthmus located ventral to the trachea connects the lobes. The right lobe is in close proximity to the common carotid artery, the internal jugular vein, and the vagosympathetic trunk; the left lobe is in close proximity to the caudal laryngeal nerve and the esophagus.

Ectopic thyroid tissue occurs frequently, located intrathoracically in the heart and heart base[3].

The parathyroid glands are closely associated with the thyroid gland, with a parathyroid gland frequently being located at the cranial surface of each thyroid lobe, and the second parathyroid gland frequently being embedded within the thyroid tissue.

Lymph drains from the thyroid gland in both cranial and caudal directions[4]. Lymph from the cranial pole of each lobe drains into the cranial deep cervical lymph node, and that from the caudal pole of each lobe flows into the caudal deep cervical lymph nodes, or directly into the venous circulation. In about 10% of dogs, there may be a lymphatic connection between the right and left lobes of the thyroid gland.

Due to the drainage pattern of the thyroid gland, ipsilateral and contralateral regional lymph nodes both cranial and caudal to the thyroid gland should be assessed for detection of thyroid adenocarcinoma or hyperplasia associated with hyperthyroidism.

Thyroid diseases include:

The major function of the thyroid gland is production of regulatory hormones, in particular thyroxine (T4.

All serum T4 is produced by the thyroid glands and T4 accounts for most of the hormones secreted by the thyroid gland, with only small quantities of T3 and reverse T3 (rT3) released.

The majority of T4 is bound to plasma proteins, and only 1% is unbound as the active form, which exerts a positive effect on cells throught the body except for the pituitary, where it exerts a negative feedback effect on TSH secretion. The function of plasma-bound T4 is to act as a reservoir and buffer to maintain a steady level of circulatory free T4. Within most cells, free T4 is deiodinated to T3 or rT3. The latter is primarily utilized during illness, starvation or excessive endogenous catabolism. Intracellular T3 binds to receptors on the mitochondria, nucleus and plasma membrane, resulting in heightened intracellular activity.

In contrast to T4, most of the T3 and rT3 are formed by deiodination of T4 in extrathyroidal sites such as the liver, kidney and muscle. Therefore, T3 levels are a poor gauge of thyroid gland function.


  1. Mayer MN & MacDonald VS (2007) External beam radiation therapy for thyroid cancer in the dog. Can Vet J 48(7):761-763
  2. Hullinger RL (1993) The endocrine system. In: Evans HE ed. Miller’s Anatomy of the Dog. 3rd ed. Philadelphia: WB Saunders. pp:559–585
  3. Roth DR & Perentes E (2012) Ectopic thyroid tissue in the periaortic area, cardiac cavity and aortic valve in a Beagle dog - a case report. Exp Toxicol Pathol 64(3):243-245
  4. Caylor HD et al (1927) Observations on the lymphatic connections of the thyroid gland. Anat Rec 36:325–333
  5. Rossetto A et al (2011) Mesenchymal chondrosarcoma of the spleen: report of a case. Tumori 97(4):e10-15