Concerned about CHG? Here is some information that you might find helpful.

What the heck is CHG and what does it have to do with my TFT?  

CHG is congenital hypothyroidism with goiter. I could go into great detail and use lots of medical terms that will probably confuse everyone (including myself), but let's make it simpler to understand (I hope).

Let's start at the beginning and define each word to help better understand.

 

Congenital: any condition present at birth.

 

Hypothyroidism: This is a bit more involved. Let's break it down.

        Hypo: underactive.
        Thyroid: a gland that produces thyroid hormones. These hormones regulate the rate of metabolism and affect growth.

Goiter: a swelling in the neck caused by an enlarged thyroid gland.

 

Now let's put it all together. Congenital Hypothyroidism with Goiter: a condition present at birth which manifests as a disease state caused by the thyroid gland producing insufficient hormones and swelling in the neck from an enlarged thyroid gland.

 

Now that we know what it is, let's discuss what effect it has on our Toy Fox Terrier population. First you need to understand that CHG is carried as a recessive disorder (not dominant). The disorder is caused by a mutation in a single gene. A Toy Fox Terrier that is a carrier of the CHG trait will show no symptoms. However, the mutated gene is present and can be passed to offspring.

 

Since the gene can be passed on, but is recessive, it needs another recessive gene to "activate" the CHG disorder. This can only happen if you breed two CHG carriers. That mating will result in approximately 25% of the litter being affected with CHG. Of the remaining 75%, 50% will be carriers and 25% will be normal. Those estimates are approximate because genetic make-up of each individual puppy is complicated and depends on a number of factors.

 

A carrier would have a genetic make-up of one dominant normal gene, we'll call it C, and one recessive mutant gene, well call it c. So it would look like this:

 

 

Let's take a look at what the mating of two carriers would produce:

 

Cc + Cc = 25% of the puppies will be CC (normal); 50% of the puppies will be Cc (carriers); 25% will be cc (affected)

 

These numbers are based on probable outcomes (for those interested, I'll illustrate the Punnett Square used to determine these outcomes at the end of this essay). It is possible, because we are allowing nature to do the gene combinations here, that a litter from two carriers would produce no CHG affected puppies . . . but should you continue to breed the two carriers together, chances are that you will at some point get a CHG affected puppy. It's like Russian Roulette . . . eventually you will get the bullet.

 

What is an affected puppy?

 

Affected puppies are usually apparent within the first few days. These puppies are smaller than their littermates. They don't actively nurse or move around much. By the time they are two weeks old, you will feel a swelling on the neck that continues to enlarge over time. Most affected puppies die or are euthanized by three weeks of age (though CHG is often misdiagnosed as "fading puppy"). If the puppy survives beyond three weeks, it will continue to be plagued with problems: dwarfism, its eyes don't open on schedule, its ear canals remain very small, and its coat is bristly. If caught early, the puppy can receive daily oral thyroid hormone medication.

 

But is it worth it? Even with the treatment, the goiter (swelling on the neck) continues to grow and restrict the airway. Not a great quality of life (and likely a short one) for any dog in my opinion.

 

 

Breeder's opinions on this vary. I am of the opinion that if a carrier possesses desirable traits not found elsewhere in your breeding program, breeding is desirable. However, if you have the desired traits elsewhere in your breeding stock in a non-carrier, there is no need to breed the carrier.

 

If you are going to breed a CHG carrier, it is critical to breed to non-carrier, or "normal" mate. Normal, or non-carrier dogs would have no recessive mutant gene, and therefore would have both genes as dominant normal. So the "normal" make-up would look like this:

 

 

Now as promised, I'll illustrate the Punnett Square. A Punnett Square is a diagram designed by Reginald Punnett to determine the probability of an offspring having a particular genotype. The square gives a summary of every possible outcome of the mating (in my examples, CHG carriers with other CHG carriers and with non-carriers). If you know which genes are recessive and which are dominant, you can use the square to determine other genetic outcomes as well (such as coat color . . . but we won't get into that). Now to the square. Let's start with the first example of two carriers (Cc) being bred. Our square would look like this:

 

This shows that one out of four (25%) puppies would be normal; two out of four (50%) would be carriers; one out of four (25%) would be affected.

 

Our square for one normal (CC) and one carrier (Cc) would look like this:

 

This shows that two out of four (50%) puppies would be normal; two out of four puppies (50%) would be carriers; no puppies (0%) would be affected.

Today it is relatively easy to avoid ever having an affected puppy. If you know your dog is a CHG carrier, don't breed that dog to another CHG carrier. If you don't know your dog's CHG status, have it tested. The price of the test is small (last I checked it was around $60) compared to the emotional and physical exhaustion of having affected puppies.

 

To see if your dog is a carrier, submit your dog's sample for testing. Instructions can be found at:

 

 

 

Hopefully this has helped you understand a bit more about CHG.

 

If you have any questions or comments about this information, please contact us.