The explanation for these swellings most often is that milk goiter is caused by the rich milk of their dam. On the surface this makes sense since the kids have goiters while nursing then the goiters will decrease in size about the same time that they are weaned. However, milk goiter can also appear in kids raised on pasteurized goat milk or milk replacer.
Some owners will panic thinking these swellings could be the first sign of CL (caseous lymphadenitis) or some other infection of the lymph nodes found in the neck.
Others will attempt to cure milk goiters by spraying a strong iodine solution on kids' tender skin around the genital area. The suggestion is that milk goiters are enlarged thyroid glands due to an iodine deficiency in a fast growing kid. According to anecdotal reports, the swellings go away within days following this treatment, although more frequently repeated iodine treatments are required to reduce the size of the throat swellings.
Iodine is an essential component of the hormones produced by the thyroid gland. These hormones called T4 and T3 are essential for cell metabolism, and growth, maintenance of connective tissue and development of the nervous system in the fetus and newborn. If there is a deficiency of iodine in the diet, these hormones cannot be produced.
From its position at the base of the brain, the pituitary gland monitors the levels of hormones in the blood. If a low level of thyroid hormones is detected, the pituitary gland sends out its own hormone called thyroid stimulating hormone or TSH, which stimulates the thyroid gland to step up production of thyroid hormones. The thyroid gland can't do this since it's missing one essential ingredient - iodine - but the pituitary gland does not know this. It keeps secreting TSH which after a time will cause the tissue of the thyroid gland to change and the entire gland will enlarge. Surprisingly, a toxic level of iodine will also cause an enlarged thyroid and hyperthyroidism due to a malfunction of the thyroid hormone producing system.
The symptoms of iodine deficiency in goats are kids which are born weak, with sparse hair coat, low resistance to stress and slow growth. Even a subclinical iodine deficiency results in small, weak kids with no obvious goiter. These symptoms do not describe the typical healthy, fast growing kid with milk goiter.
Mild enlarged thyroid glands in the goat it is not easy to detect because the swelling is located behind the larynx which would be mostly out of our sight, and slightly below. Milk goiter is quite obvious, appearing in front of, and on both sides of the throat over the larynx.
True iodine deficiency is rare in our supplemented goats, but soil in the Northeast US is low in iodine so it may occur in that area in goats that are not provided a supplement. If the dam has an adequate supply of iodine, her milk will contain enough to support her nursing kids. If the dam was deficient she would exhibit common symptoms of iodine deficiency such as abortion. The fetus cannot survive to birth without adequate levels of this mineral. In fact in third world countries where iodine deficiency is a problem, the most serious sign is natural abortion.
On the other hand, the fetal thyroid gland is very sensitive to iodine toxicity. Kids born to dams with high levels of iodine in their system are born with obvious thyroid enlargements.
Assuming that this was an iodine deficiency, the owner's veterinarian prescribed potassium iodine administered by mouth but this did not result in a response. Analysis of feed showed an adequate amount of iodine. Blood and milk samples showed that rather than a deficiency the kids and dams had a slightly higher than normal level of iodine.
One kid was sacrificed and a necropsy was performed to determine the cause of the swellings. Results showed that the throat swelling were due to "gross enlargement of the thymus, with most of the extra thoracic portion - weighing in excess of 200 grams - being located in the upper neck region." Histologically, the thymus tissue was normal. Two other kids were also post-mortemed and showed similar thymus enlargements. In one, a 4 month old kid which had some regression, the thymus weighed nearly 170 grams, and in another younger kid the thymus enlargement was located under the lower jaw area.
Goats of all ages from the entire herd were screened for diseases including CAE, enzootic bovine leucosis, border disease, infectious bovine rhinotracheitis, Toxoplasma gondii, and other bacteriology, virology, haematology, biochemistry and serology tests. The results of all were negative. The reason for the thymic enlargement at that time was unresolved.
A type of immune cell which originates in the bone marrow is processed by the thymus gland. This immune cell, called a T-cell (thymus-dependent (T) lymphocyte), must be able to recognize the difference between a foreign invading cell and the body's own cells. These cells receive their training in the thymus gland by a process which is one of the great mysteries of science.
The thymus of the goat can be quite large in young animals reaching a peak in size at about 4 months of age, then begins to regress to a small size by 6 to 7 months of age. In castrated males the thymus enlargement can persist for some time longer.
It is known from studies of genetic diseases in which the thymus fails to develop or from surgical removal of the thymus gland in young children that the result will be permanent immuno-deficiency.
It should be noted that iodine can have toxic effects at high levels. It is easily absorbed through the skin especially if repeated over large areas of intact skin or to absorptive mucous membranes. Iodine toxicity is more likely if the kid already had a normal iodine level.
Milk goiter is the very common, normal enlargement of the thymus gland which is part of the maturation of the immune system in nearly all juvenile mammals. Why the thymus gland is larger in some kids or children than others is not well understood. However, reducing the size at an early age could result a less efficient immune response for the life of the goat as has been shown to be the case in humans. Instead, welcome milk goiters with the understanding that the immune system is laying the foundation for your kids' healthy future.
1. Pritchard, G.C. (1988) Throat swellings in goats. Goat Vet. Soc. J., 10 (1), 34-7