Congenital Hypothyroidism French Bulldog Type (CHG)
Congenital Hypothyroidism French Bulldog Type (CHG) is a congenital disorder of thyroid gland. In general, congenital goiter is an enlargement of the thyroid gland. This enlargement may cause decreased, increased or normal secretion of the thyroid hormone. In hypothyroidism, the secretion of the thyroid hormone is decreased, which causes many consequential symptoms. Generally, hypothyroidism occurs most commonly in dogs, but can develop also in other species, such as cats or horses. In humans, other than hereditary, congenital goiter may be caused also by dyshormonogenesis, transplacental passage of maternal antibodies or transplacental passage of goitrogens.
The thyroid gland is one of the largest endocrine glands in the dog’s body. It is responsible for controlling the metabolism in general: rate of use of energy sources, protein synthesis and regulations regarding other hormones. This all processes are regulated by production of thyroid hormones, which regulates the growth and rate of function of many other body systems. For proper thyroid function iodine is essential.
Characteristics and Symptoms
Usually hypothyroidism develops in adult age of dog, but sometimes it can also occur in neonatal period. As well as in humans, early occurrence of goiter signals hypothyroidism caused by dyshormonogenesis. Congenital Hypothyroidism French Bulldog Type (CHG) causes slow physical and mental development and other symptoms collectively known as cretinism, and other metabolic abnormalities. Common symptoms shown by CHG affected puppies are delayed opening of eyes and ear canals, poor nursing, inactivity, unresponsiveness to environmental stimuli, hypomyelination of the central nervous system, and disproportionate dwarfism caused by epiphyseal dysplasia and macroglossia.
Congenital Hypothyroidism in dogs is mostly a hereditary disorder of metabolism but there can be also other causes of its occurrence, such as late fetal or neonatal iodine excess or deficiency or also consumption of variety of drugs in neonate or pregnant dam.
Congenital Hypothyroidism with goiter (CHG) has been identified also in several other breeds, such as toy fox, rat, and Tenterfield terriers and in Spanish water dogs. Although the symptoms appear to be similar, in each of these breeds has been detected a breed-specific mutation in the TPO (thyroid peroxidase) gene that causes CHG. Mutations in the same gene are also the most common cause of CHG in human patients. Thyroid peroxidase protein is necessary for synthesis of thyroid hormone. When there is a lack of TPO in the organism, iodide is not able to incorporate into thyroglobulin, and consequently failure of thyroid hormone synthesis occurs.
Congenital Hypothyroidism (CHG) French Bulldog Type is inherited in an autosomal recessive pattern. Heterozygotes have no symptoms. Dogs homozygous for the mutation will display the CHG symptoms. At conception, each cub has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.