THYROID FUNCTION IN RACING & BREEDING THOROUGHBREDS

Dr McGregor's Equine Blood Analysis Series

Thyroid Function Testing, and effects of thyroid deficiency are an emerging series of interests in aximising racing and breeding potential.
Thyroid malfunction has been recognised for centuries through the observed changes in the size of the thyroid gland. Enlarged thyroids have been labelled as goitres, and are often seen during periods of coat ill-thrift. Good observation has also tied thyroid function ups and downs to the seasons. This has been particularly important for the breeding industry with our aim for early foals, as thyroid function is low in the end of winter period when we want mares to cycle.
Firstly, let's examine the normal changes in thyroid function during a year. Hormonal production from the thyroid gland is tied to the amount of daylight, so function is at a peak in the southern hemisphere in late December and at its lowest point during late June and July. In nature, prior to human intervention, horses migrated with the seasons following the Spring. Thus, they maintained good thyroid functions by not going into full winter anywhere. We have changed this by holding horses at particular latitudes and thus they go through the full thyroid gamit for high and low functions. Over the years, good trainers have observed the advantages of 'wintering' horses in summer areas eg. Tesio with southern and northern Italian stables and, closer to home, Bart Cummings and Tommy Hughes with Melbourne and Brisbane stabling of horses.
Low thyroid function causes a large number of problems for the trainer and the breeder. For the trainer, low thyroid function will cause reduced bone density with young horses, causing more rapid concussion problems such as shin soreness, carpitis and feet soreness. With this we also have pain which is likely to affect appetite and attitude to work. Low thyroid affects oxygen uptake in muscles which can lead to myopathies with tying up or cramping, particularly fillies. General ill-thrift with facial hair loss or failure to drop coats are also common findings. Work can affect thyroid levels. Irritability is another common finding, either primarily due to effects on the nervous system or secondly, to bone and muscle pain.
The breeder finds thyroid function important both for stallion and mare fertility. From the stallion point of view, low thyroid gives very poor sperm production. In the last few years, our practice has treated three infertile stallions with thyroid deficiency and each has returned to relatively normal function. For stallions, treatment must begin three months before sperm is required. For mares, lights have been used to stimulate thyroid function for many years (also stallions in Europe and New Zealand) and the use of thyroid as a feed supplement is relatively common in mares needing to be bred in early February in the northern hemisphere or September in the south.
This article is not aimed to get everyone to test for thyroid function, but to suggest considering it in diagnosis of problems facing both racing and breeding horse-people, and when you see the patchy-coated filly in August who won't put on condition, doesn't want to work and won't eat properly. Check thyroid function and, if it is low, watch her change in less than two weeks with treatment.