Case Report: Diet-associated iodine-deficiency goitrous hypothyroidism in a dog: a longitudinal clinical, endocrine, and ultrasonographic evaluation.

Iodine-deficiency hypothyroidism is well recognized in humans, but is considered rare in dogs, largely because commercial canine diets typically meet the established iodine requirements. Herein, we describe a dog that developed primary hypothyroidism with bilateral goiter attributable to dietary iodine deficiency. A 9-year-old spayed female Chihuahua fed a non-standardized home-prepared diet without vitamin and mineral supplementation presented with mild weight gain and bilaterally enlarged thyroid lobes, but without the classic clinical signs of hypothyroidism. Basal thyroid hormone testing revealed markedly decreased total and free thyroxine concentrations with pronounced elevation of thyroid-stimulating hormone levels, consistent with primary hypothyroidism. Ultrasonography revealed symmetric thyroid enlargement with preserved architecture and mild diffuse hypoechogenicity. Fine-needle aspiration excluded neoplasia, and the serum inorganic iodine concentration was severely decreased. The dog's diet was changed to a commercially prepared, iodine-supplemented formula without thyroid hormone replacement. Over the following 2 months, serum thyroid hormone concentrations normalized, serum iodine concentrations returned to reference values, and ultrasonographic abnormalities improved markedly, with a substantial reduction in thyroid size and restoration of homogeneous echogenicity. This case highlights the potential for iodine-deficiency hypothyroidism to occur in dogs consuming unbalanced home-prepared diets and demonstrates the reversibility of functional abnormalities and marked ultrasonographic improvement following restoration of adequate dietary iodine intake.