Equine hypothyroidism: the long term effects of thyroidectomy on metabolism and growth in mares and stallions.

The research investigated the long-term effects of surgical removal of the thyroid gland (thyroidectomy) on the growth and metabolism of young horses, discovering a notable impact on growth, feed consumption, body temperature, and other physiological markers, but these effects were mitigated to a degree by supplementing with thyroprotein.

Study Design and Setup

• The study involved 11 yearling grade horses, with 6 of them (3 male, 3 female) undergoing a surgical procedure known as thyroidectomy (Thx), which involves the removal of the thyroid gland. The remaining 5 horses (3 male, 2 female) served as controls for the experiment.
• The researchers monitored and compared the conditions of the Thx horses and the control horses over a period of 67 weeks. Features that were measured include heart rate, body temperature, feed intake, body size, blood health markers (such as cholesterol and calcium levels), and developmental factors (like growth plate closure and tooth eruption times).

Results of Thyroidectomy

• Post-surgery, the horses that underwent Thx showed no growth in height, and displayed a sensitivity to cold temperatures. They additionally exhibited a slow shedding of their winter coats and had hair that looked dull and coarse.
• The horses also showed behavioural changes, appearing more docile, lethargic, and with a sleepy expression. Edema, or swelling, was observed in the rear limbs, and the faces looked noticeably coarser and thicker.
• The horses’ normal body temperatures dropped significantly, and their cholesterol levels rose correspondingly. The amount of feed they consumed was less than the controls, as was their body weight gain – the difference was particularly noticeable in females, who ate 46% less and gained 47% less weight compared to the control group.
• The levels of calcium in the blood were unchanged after the surgery, though levels of phosphorus decreased significantly, especially among the female horses. The closure of the growth plates and the eruption of incisor teeth both experienced delays.

Effects of Thyroprotein Supplementation

• During weeks 46 to 53 of the experiment, the male horses who had undergone thyroidectomy were fed a thyroprotein supplement. After the supplementation, the signs of hypothyroidism were dramatically reversed.
• Once the supplementation was stopped, symptoms of hypothyroidism returned, but generally to a lesser degree than those seen prior to the supplementation. The only exception was the value of T (possibly referring to thyroid hormone levels), which fell back to non-detectable levels after cessation of thyroprotein supplementation.