Topography
Each lobe rests on the lateral surface of the trachea, caudal to the larynx and medial to the common carotid artery. In the cat the two lobes are normally not joined by an isthmus. Each lobe has two parathyroid glands: the external one at the cranial pole and the internal one embedded in the thyroid tissue. Accessory (ectopic) thyroid tissue can occur anywhere from the base of the tongue to the heart base.
Clinical relevance
- Hyperthyroidism: the most common endocrinopathy of older cats, almost always due to adenomatous hyperplasia or a functional adenoma; in most cases (≈70%) both lobes are affected, and carcinoma accounts for less than 3%. It causes weight loss with increased appetite, tachycardia, hypertension and secondary cardiac hypertrophy, and it can mask chronic kidney disease by raising the glomerular filtration rate.
- Palpation: in a healthy cat the lobes are not palpable. A thyroid nodule is sought by sliding the fingers along both sides of the trachea from the larynx to the thoracic inlet; enlarged, loosely attached lobes can descend towards the thoracic aperture.
- Ectopic tissue: hyperfunctional accessory thyroid tissue, sometimes intrathoracic, explains some failures of thyroidectomy. Technetium-99m pertechnetate scintigraphy locates all active tissue.
- Treatment: methimazole or carbimazole, an iodine-restricted diet, radioiodine (I-131, the curative treatment of choice) or thyroidectomy, which must preserve at least one external parathyroid gland to avoid hypocalcaemia.
3D model
Schematic shape (illustrative fidelity), placed from landmarks on the skeleton and the scanned organs. It shows topography, not fine morphology.