Overview
The thyroid gland (glandula thyroidea) produces the thyroid hormones (thyroxine and triiodothyronine), which regulate metabolism, growth and fetal development, and calcitonin. Their synthesis needs dietary iodine.
Topography and shape
- Lobes: two, oval, smooth and firm, one on each side of the first tracheal rings, caudal to the larynx.
- Isthmus: in the horse it is a narrow band of connective tissue, sometimes almost absent, crossing the ventral surface of the trachea; unlike in cattle, it has no glandular tissue.
- Relations: they are covered by the neck muscles and the skin of the throatlatch, close to the common carotid artery, the vagosympathetic trunk and the jugular vein; the oesophagus lies dorsal to the trachea. The cranial parathyroid glands usually lie next to the thyroid.
Clinical relevance
- Palpation: in the horse the lobes can usually be felt in the throatlatch region, especially in lean animals, which makes enlargement easy to detect.
- Thyroid adenoma: common in older horses; it is usually unilateral and non-functional, and is found as a lump in the throatlatch.
- Congenital hypothyroidism and dysmaturity: foals with mandibular prognathism, incomplete ossification of the carpal and tarsal bones, ruptured extensor tendons and weakness. It has been associated with diets of the pregnant mare high in nitrates or low in iodine.
- Goitre: both too little and too much iodine in the mare's diet, for example from seaweed supplements, can cause goitre in the foal.
- Adult hypothyroidism: the evidence that it exists as a clinical disease is weak; low T4 concentrations are common in non-thyroidal illness or after phenylbutazone and are not enough for a diagnosis.