In early June 2015, a sheep producer from a property about 30 km south west of Bathurst found a single second cross lamb with a massive swelling of the ventral neck. The lamb survived for two days but then died. The lamb was from a mob of approximately 600 lambing mature first cross ewes running on improved pastures on sedimentary rock derived soils.
A single two-day-old lamb presented for examination on 4 June 2015 with a swelling of the ventral neck extending from the submandibular region to the thoracic inlet.
The histologic section consists of multiple, variably sized cystic follicles which are empty and collapsed, or lined by attenuated epithelium or single to multiple layers (hyperplasia) of differentiated cuboidal epithelium. Cystic follicles often contained pale, fibrillar eosinophilic material (colloid). These well-differentiated follicular cysts are supported by minimal to moderate fibrocollagenous stroma. Areas in this section resemble embryological ultimobranchial gland (which generate C cells of the thyroid gland). Histology of the neck mass was suggestive of thyroid tissue. The macrofollicles were distended with colloid, tested to be Periodic Acid Schiff’s (PAS) stain positive, confirming that the tissue was thyroid. The specific gravity of the fluid was 1.018, which is consistent with a colloidal exudate. Histopathology of the thyroid was therefore consistent with the cystic form of goitre (colloidal goitre).
While goitre is occasionally seen in lambs and kids on the central tablelands of NSW and outbreaks were reported in both lambs and kids in the winter and spring of 2015, the owner of this flock saw no cases of goitre during that period and had not seen it prior to this case.
Findings of diffusely enlarged thyroid lobes are consistent with diffuse hyperplastic goiters encountered in neonates that die at or soon after birth. Newborn goitrous lambs with hyperplastic goitre (congenital hypothyroidism) are likely born to dams on iodine–deficient diets, although affected dams may not display clinical hypothyroidism. The principal pathogenic mechanisms involved in the development of thyroid hyperplasia include iodine-deficient diets, goitrogenic compounds that interfere with thyroid hormone synthesis, dietary iodide excess and genetic enzyme defects in thyroid hormone synthesis.
Outbreaks of iodine deficiency are sporadic. It can occur from high dietary intake of calcium that reduces intestinal absorption of iodine, pasture heavily laden with lime, or when permanent pasture is plowed and resown with different plant species. Severe thyroid hyperplasia can also result from marginal iodine-deficient diets containing the following goitrogenic or antithyroid compounds:
Colloid goiter, typically seen in sub-adults and adults, represents the involutionary phase of diffuse hyperplastic goiters. These may develop either after sufficient iodine has been added to iodine-deficient diets of animals with hyperplastic goiter, or after the reduced demands of thyroid hormones in sub-adults. Colloid goitre occurs due to continued production of colloid by hyperplastic follicular cells irrespective of decreased colloid endocytosis associated with diminished thyroid stimulating hormone (TSH) levels from the return of blood thyroxine and triiodothyronine to normal. The reduced vascularity and development of colloid-distended macrofollicles manifest grossly as diffusely enlarged thyroid lobes which are more translucent and lighter in colour than hyperplastic goitres. Histologically, large macrofollicles containing colloid are lined by flattened inactive cells rather than the actively secreting follicular cells associated with endocytotic vacuoles. Follicles can become involuted with papillary projections of remnant follicular cells into their lumen. Interfollicular capillaries are poorly developed due to the lower TSH levels.
Differential diagnoses in this case include thyroglossal duct cysts and ultimobrachial cysts. Thyroglossal duct cysts are incidental findings in dogs and may form thyroid microfollicles. In sheep ultimobranchial cysts are common in the hilar portions of thyroids. These are lined by squamous epithelium and have a watery or pasty content due to degenerated desquamated epithelial cells.
In this case, the cysts resembled thyroid macrofollicles containing pale colloid-like eosinophilic material. The cystic nature of the thyroid mass was due to the development of macrofollicles distended with colloid (confirmed to be PAS positive), which is consistent with a colloid goitre. There were mixed histological features of ultimobranchial gland, hyperplastic goitre and colloid goitre in this mass, which made this case particularly interesting.