History
A juvenile laboratory-reared female sheepshead minnow (Cyprinodon variegatus), a euryhaline fish species native to eastern pan-American coasts from Cape Cod to French Guiana, was euthanized by immersion in 500 mg/L of tricaine methane sulfonate (MS-222) buffered 1:1 with sodium bicarbonate and submitted for necropsy to the Aquatic Pathology Service at the University of Georgia. Clinical signs were limited to a mass protruding from the left opercular cavity of unknown origin. The single affected individual was part of a breeding colony housed in aquariums with recirculating artificial seawater (13-ppt salinity) cultured by a commercial toxicology test facility for bioassay purposes.
Clinical and Gross Findings
The female, at an overall length of 3.7 cm, was in good physical condition with an approximately 0.4-cm soft, reddish-brown mass protruding ventrally from the left opercular cavity in association with the bases of multiple gill arches (Figure 1). There were no additional gross abnormalities. The body was decalcified, serially sectioned transversely, and processed routinely, followed by staining with H&E for histopathologic evaluation.
Gross photograph of the left side of a juvenile female sheepshead minnow (Cyprinodon variegatus) that was submitted for necropsy with a soft, reddish-brown mass protruding ventrally from the left opercular cavity. White bar = 1 cm.
Citation: Journal of the American Veterinary Medical Association 262, 9; 10.2460/javma.24.04.0260
Histopathologic Findings
The opercular cavity mass arose from the ventral subpharyngeal region, which was markedly distended by an expansile mass of hyperplastic thyroid tissue obscuring the ventral aorta and bilaterally displacing the bases of the gill arches (Figure 2). The mass was composed of variably sized, small to large, well-differentiated follicles forming multiple pseudoencapsulated nodules. Follicles were lined by either attenuated or columnar epithelial cells and were filled by homogenous, deeply eosinophilic colloid. The columnar cells had predominantly basilar ovoid nuclei with marginated chromatin and a central nucleolus. The apical cytoplasm was frequently vacuolated and contained an eosinophilic colloid droplet. Adjacent luminal colloid was often vacuolated (Figure 3).
Photomicrograph of the subpharyngeal region of the sheepshead minnow described in Figure 1. A multinodular expansile mass of well-differentiated hyperplastic thyroid tissue containing variably sized colloid-filled follicles (multinodular goiter) expands the subpharynx and impinges bilaterally on gill arches (arrows). H&E stain; black bar = 500 µm.
Citation: Journal of the American Veterinary Medical Association 262, 9; 10.2460/javma.24.04.0260
A higher-magnification photomicrograph of the hyperplastic thyroid tissue described in Figure 2. Colloid-filled follicles variably lined by active columnar or flattened, atrophic epithelial cells form an unencapsulated, expansile mass that impinges on the base of a gill arch. The columnar epithelial cells frequently exhibited endocytosis in association with adjacent vacuolated colloid. H&E stain; black bar = 10 µm.
Citation: Journal of the American Veterinary Medical Association 262, 9; 10.2460/javma.24.04.0260
Morphologic Diagnosis and Case Summary
Morphologic diagnosis: chronic, severe, multinodular thyroid follicular cell hyperplasia.
Case summary: multinodular goiter in a sheepshead minnow.
Comments
This sheepshead minnow’s gross and histological lesions are consistent with thyroid follicular cell hyperplasia or goiter, a term commonly applied to nonneoplastic and noninflammatory thyroid gland enlargements. Goiter is a common condition of captive and occasionally wild fish resulting from iodine deficiency, though their etiologies may differ from goiters encountered in mammals. In teleosts, thyroid tissue is nonencapsulated, with follicles scattered predominantly around the ventral aorta and branchial arteries ventral to the pharynx. In some species, ectopic follicles commonly occur in other anatomic locations, including the cranial kidney, spleen, and heart, as well as retro-orbitally.1 Due to the lack of encapsulation and occurrence of ectopic tissue, hyperplastic lesions can appear invasive and must be differentiated from true neoplasia. Both thyroid adenomas and carcinomas have been reported in fish, and distinguishing between these conditions is essential to treatment, prognosis, and population management strategies.2
Thyroid hyperplasia can be categorized as simple (diffuse), nodular or multinodular. Simple hyperplasia involves enlargement of the entire gland, while nodular or multinodular hyperplasia occurs when 1 or more discrete nodules are present, respectively, among relatively normal follicular tissue. Hyperplasia of ectopic follicles can also occur, complicating differentiation from metastatic neoplasia. Follicular cell hyperplasia (hyperplastic goiter) occurs when iodine levels are insufficient, resulting in increased endocytosis of colloid. Follicles containing pale vacuolated colloid vary in size and shape, and some collapse. Follicular lumens are lined by columnar cells with hyperchromatic basilar nuclei. The changes observed in diffuse hyperplastic and colloid goiter are consistent throughout the thyroid tissue. A nodular goiter, as seen in this case, results from alternating periods of hyperplasia and colloid involution, with fluctuating iodine levels and microscopic features of both seen within nodules. If iodine and thyroid hormone levels return to normal, follicular epithelia regress. The lumens distend with colloid due to decreased thyroid-stimulating hormone–stimulated colloid endocytosis, and the resulting macrofollicles become lined by attenuated, atrophic follicular cells.2
A study2 of fish thyroid lesions characterized follicular cell hyperplasia as distinguishable from discrete adenomas based on criteria including cellular atypia, structural complexity, pseudoencapsulation, and the presence of particular growth patterns (papillary or solid). In contrast, follicular cell carcinomas have anaplastic features, including cellular and nuclear atypia, high mitotic rates, and disorganized growth. Of note, metastasis was not described as a criterion of neoplasia or malignancy, although it can occur with follicular cell carcinomas, which may or may not present as discrete masses.2
Fish with thyroid hyperplasia can show opercular flaring, increased respiratory effort, and hyporexia, often accompanied by a visible mass in the opercular or pharyngeal regions. Clinical diagnosis is usually presumptive but may be aided by measurement of water iodide and nitrate concentrations or based on response to treatment.1 Plasma T4 may be low, but reference ranges must be validated for each species, limiting clinical usefulness. Similarly, T3 and thyroid-stimulating hormone are usually low, but assays may be unreliable.1 Histopathologic diagnosis based on the criteria described above is confirmatory.
Iodine is the essential mineral required for thyroid hormone synthesis. In mammals, iodine is obtained from dietary sources, while in fish, it is acquired primarily from water across the gill epithelium and less efficiently by the alimentary tract.3 In water, iodine exists as iodide (I–), iodate (IO3–), iodine (I2), and dissolved organic iodine. While iodide and dissolved organic iodine are bioavailable to fish, iodine and iodate are not.1,4 In artificial systems, iodide levels in water should be regularly measured and maintained in the range of 0.03 to 0.06 mg/L (note that this is not a measurement of total iodine).1
Potential causes of iodine deficiency unique to artificial fish-rearing systems include varying levels of iodine between commercial brands of salt mixtures used to create artificial seawater, consumption of goitrogenic plants, the use of ozone for water clarification and disinfection, and chronically elevated water nitrate levels. In a report4 describing an epizootic of goiter in multiple marine fish species at a public aquarium, the cause was traced to a change in the brand of salt mix used in seawater makeup. The brand in question was determined to contain only a fifth of the iodine provided by the former product. No new cases occurred when iodine concentrations were returned to former normal levels.4 The feeding of goitrogenic plants, such as raw brassicas, has also been indicated as a possible cause of goiter in fish.1
Public aquaria and aquaculture facilities frequently apply ozone in recirculating water systems to facilitate water reuse by removing organic material and potential pathogens. However, ozone is known to affect iodine availability by oxidizing iodide and dissolved iodine to biologically unavailable iodate.3,5 As a result, regular measurement of iodide levels and supplementation as needed are important to maintaining thyroid health.
Another complication of water recirculation linked to goiter development is the buildup of nitrate, the end product of bacterial nitrification in aquatic systems wherein toxic ammonia and nitrite are metabolized to nitrate. Nitrate levels above 70 mg/L, possibly lower, inhibit the uptake of iodide presumptively across the gills and intestine or into the thyroid itself, leading to deficiency.5
Thyroid follicular cell hyperplasia or goiter is a frequently encountered and potentially multifactorial condition in aquarium fish. While detailed history, husbandry conditions, and water quality parameters were not made available for assessment by the submitter, gross and histopathologic findings in this sheepshead minnow were consistent with multinodular goiter. The material presented highlights the importance of complete history taking, which includes diet and water quality information, in captive fish settings. In the case of goiter, an appropriate histological characterization can support whether treatment is likely to be successful and guide further husbandry and environmental investigation into potential causes of the condition. Such characterization helps avoid misdiagnosis and subsequent avoidable depopulation or euthanasia of affected individuals.
Acknowledgments
None reported.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
The authors have nothing to disclose.
References
- 2.↑
Fournie JW, Wolfe MJ, Wolf JC, Courtney LA, Johnson RD, Hawkins WE. Diagnostic criteria for proliferative thyroid lesions in bony fishes. Toxicol Pathol. 2005;33(5):540-551. doi:10.1080/01926230500214509
- 3.↑
Sherrill J, Whitaker BR, Wong GTF. Effects of ozonation on the speciation of dissolved iodine in artificial seawater. J Zoo Wildl Med. 2004;35(3):347-355. doi:10.1638/03-025
- 4.↑
Jalenques M, Vergneau-Grosset C, Summa N, Youcef WA, St-Cyr J-F, Lair S. A cluster of cases of thyroid hyperplasia in aquarium-housed tropical marine teleosts following a change of salt mix brand. J Zoo Wildl Med. 2020;51(3):725-728. doi:10.1638/2019-0221
- 5.↑
Morris AL, Stremme DW, Sheppard BJ, Walsh MT, Farina LL, Francis-Floyd R. The onset of goiter in several species of sharks following the addition of ozone to a touch pool. J Zoo Wildl Med. 2012;43(3):621-624. www.jstor.org/stable/41681883 doi:10.1638/2010-0160R2.1
