Thyroid gland tumours in the horse: clinical diagnostic, treatment and histological characterisation

Reasons for performing study:

Tumours of the thyroid gland are rare in the equine species. In literature varying clinical signs and diagnostic findings are described in single reports and case series. Reported clinical signs are related to upper airway compression or thyroidal metabolic activity. A considerable risk of laryngeal hemiplegia is described when horses were treated with hemithyroidectomy [1].

Objectives:

To review 11 horses referred with a unilateral thyroid neoplasia in order to describe clinical presentation, diagnostic findings, treatment and outcome. Histopathology was performed on all resected thyroid lobes.

Study design:

Retrospective case series.

Methods:

Records from horses with histologically confirmed thyroid tumours treated with unilateral thyroid lobe resection were reviewed (2003–2013). Short-term outcome was defined for 2 weeks after surgery. Owners’ questionnaire was performed for evaluation of long-term outcome defined as a minimum of 6 months.

Results:

Affected horses were aged between 6 and 21 years. In contrast to literature the majority of tumours were adenocarcinomas (9/11), mainly found in mares (8/9). Histology revealed follicular and parafollicular (C-cell) mixed growth pattern of the carcinomas. Tumour size, growth rate and sonographic appearance did not correlate to histological characteristics. Ventral displacement of the thyroid lobe and close preparation along the thyroid capsule during surgical resection are inevitable to avoid damage to laryngeal nerves. Seroma formation was the only observed short-term complication. No long-term complications occurred.

Conclusions:

Hemithyroidectomy is regarded the treatment of choice in horses with a unilateral thyroid lobe neoplasia with excellent long-term prognosis. High incidence of malignant tumours particularly in mares has to be considered. Tumour characteristics can only be derived from histopathology and immunhistochemistry.