Morphometry, Topography and Arterial Supply of the Thyroid Gland in Brazilian Shorthair Cats

Abstract

Background: Thyroid gland diseases are the most common endocrinopathies in feline practice. Diagnosis and surgical treatment must base on solid anatomical knowledge about the gland size, localization, and blood supply. However, some textbooks provide a general anatomical description of the thyroid gland of domestic carnivores. Thus, specific details of the feline gland are missing. The present study aimed to investigate the dimensions, topography, and arterial supply of the thyroid gland in Brazilian shorthair cats and, therefore, provide additional data to diagnose and treat feline thyroid diseases.

Materials, Methods & Results: Thirty Brazilian shorthair cats formalin-fixed cadavers (15 male and 15 female) were injected with red-stained latex solution by a canula in the thoracic aorta. The necropsy unit of the Rural Federal University of Rio de Janeiro donated the specimens. The study included only adult animals with no history of thyroid disease. After the fixation period, the cadavers were dissected to investigate the measurements (length, width at cranial and caudal poles, and thickness), topography, and in situ arterial supply of the thyroid lobes. The mean measurements of the length, cranial pole width, caudal pole width, and thickness in the right lobe were 19.39 ± 3.10 mm, 5.36 ± 1.40 mm, 3.67 ± 0.93 mm, and 1.30 ± 0.29 mm, respectively; and 20.29 ± 3.35 mm, 4.85 ± 1.58 mm, 3.88 ± 0.91 mm, 1.64 ± 0.65 mm in the left lobe, respectively. There were no statistical differences (P > 0.05) in the comparison of the measures between sexes or antimers (sides). Pearson's linear correlation detected a positive, moderate (r = 0.55), and significant (P < 0.05) correlation between the right and left lobe lengths. In 70% of the cats, both left and right lobes had the cranial poles located at the same level. Typically, the lobes extended between the first to the eighth tracheal ring. However, the cranial pole of some lobes located as cranially as the cricoid cartilage level, and the caudal pole as caudally as the 12th tracheal ring. Fifty-six percent of the cats had a ventrally located isthmus. In all the sampling, one single thyroid artery emerged as a branch of the common carotid artery and provided branches directly to the thyroid lobe, isthmus and the adjacent muscles and esophagus.

Discussion: Besides establishing average dimensions of normal thyroid lobes in Brazilian shorthair cats, this study detected no significant difference between the average measurements of right and left lobes. Also, a positive linear correlation between the length and width of the right and left lobes became evident. Therefore, the practitioner must consider suspicious any length asymmetry between right and left thyroid lobes until further endocrine test proves otherwise. Most of the cats had the right and left thyroid lobe positioned at the same transversal level; however, positional asymmetries are not uncommon. Unlike dogs, Brazilian shorthair cats have only a single artery to supply each lobe: the thyroid artery. In a feline thyroidectomy, the surgeon must avoid blindly ligating the thyroid artery since this vessel also provided numerous branches to adjacent muscles and esophagus. In a bilateral thyroidectomy, the ventral region between lobes should be thoroughly inspected for the common presence of an isthmus. Sometimes, the surgeon may need to extend the incision caudally beyond the 12^th tracheal ring level to visualize the gland tissue entirely.

Keywords: endocrinology, feline anatomy, morphometrics, thyroidectomy.