Also known as: Thyroid Hyperplasia
Introduction
The thyroid in fish is unencapsulated and diffuse in nature, located along the floor of the gill chamber. Ectopic follicles often occur in the spleen, the kidney and other organs.
Iodide is required for the synthesis of thyroid hormones.
Goitre is the most common endocrine disorder reported in fish and has been reported in a large number in freshwater, seawater or brackish water, in captive and natural environments.
Iodide deficiency leads to a decrease in thyroid hormone production which stimulates an increase in TSH which leads to a growth in size and number of thyrocytes, which appears as a goiter.
Iodide is obtained both from the diet and from the water in fish and iodide deficiency can induce hypothyroidism and goitre in extreme circumstances.
Certain chemicals can impair the incorporation of iodide into the thyroglobulin molecule. These goitrogenic chemicals, such as glucosinolates found in cassava, the cabbage family and canola meal interfere with iodide and impair thyroid hormone synthesis.
Sometimes the ability to take up iodide from environmental sources is impaired, for example by nitrites.
Certain metabolic byproducts of bacteria may also be goitrogenic and account for goitres in fish held in captivity with filter systems relying on bacterial action.
Clinical Signs
Fish will exhibit a large bilateral nodular swelling at the base of the gills extending upwards along the gill arches.
Diagnosis
Clinical signs are characteristic.
Histological examination reveals thyroid hyperplasia which appears invasive.
It may also appear metastatic due to the ectopic follicles often found in the spleen, kidney and other organs. This does not indicate that the lesion is neoplastic.
Thyroid hyperplasia may be difficult to distinguish from thyroid carcinoma.
Treatment
Iodine replacement therapy is beneficial if iodide deficiency is suspected.
Goitrogenic substances in the feed should be sought out and removed.
Thyroxine has been used to restore the negative feedback on TRH and TSH production and has been successful in treating the goitre.
The condition is often reversible if the cause of the goitre can be identified and removed.
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References
Leatherland, J. (2006) Protozoan and Metazoan Infections, Volume 2 CABI
Lewbart, G. (1998) Self-assessment colour review of Ornamental Fish Manson Publishing
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