The development of hypothyroidism in young animals is fairly rare and often due to congenital primary disorders such as thyroid dysgenesis (aplasia, hypoplasia or ectasia) and dyshormonogenesis (defects in the synthesis of thyroid hormones). Forms of congenital secondary hypothyroidism are, instead, related to congenital malformations of the pituitary gland which are accompanied by deficiencies not only of thyroid-stimulating hormone (TSH) but also of other hormones, in particular of growth hormone; this gives rise to a condition known as pituitary dwarfism or panhypopituitarism. Unfortunately, the incidences of these disorders are not currently known since most affected puppies die early before being weaned. Forms of acquired hypothyroidism are uncommon in puppies and usually the consequence of a lack of iodine in the food (for example, diets consisting exclusively of meat) or the administration of goitre-promoting substances (e.g., sulphonamide).
SIGNALMENT AND CLINICAL SIGNS
The presence of thyroid hormones together with growth hormone and insulin-like growth factor (IGF I), which act in synergy, is of fundamental importance for normal growth in young animals. The characteristic clinical signs of the juvenile form of hypothyroidism are delayed skeletal growth associated with defective development of normal cognitive function, the reason why the pathology has also been named cretinism. The presence of this endocrine disorder is manifested already in the first few weeks of life, but in the eyes of the owner the problem becomes clearly evident when the puppy is about 8-12 weeks old. Dogs affected by congenital hypothyroidism, unlike those with pituitary dwarfism, appear to be out-of-proportion dwarfs, with a large, broad head and shorter than normal limbs and trunk (Figs. 1A and 1B). These developmental abnormalities are manifestations of epiphyseal dysgenesis and of growth retardation: the signs are also evident on X-rays. The physical changes that characterize the juvenile form of hypothyroidism include: macroglossia, delayed dental eruption and the persistence of the coat typical of puppies which, as time passes, becomes thin and shaggy because it is not replaced by the adult coat. Compared to healthy subjects, hypothyroid puppies show signs of mental retardation and are lethargic and weak: an examination of the neuromuscular system may show signs of joint laxity, muscle tremors and decreased reflexes. Gastrointestinal manifestations associated with cretinism are lack of appetite and constipation.
Goitre, which is the clinical manifestation of excessive secretion of TSH, is present in cases in which the congenital forms are due to a defect in the synthesis of thyroid hormones or, in cases of acquired hypothyroidism (insufficient intake of iodine or assumption of goitre-promoting substances).
LABORATORY FINDINGS
The laboratory changes found in juvenile hypothyroidism are the same as those in acquired primary hypothyroidism: a mild, non-regenerative, normochromic, normocytic anaemia and fasting hypertriglyceridaemia and hypercholesterolaemia. There may also be mild hypercalcaemia in some cases of congenital hypothyroidism.
DIAGNOSTIC IMAGING
Radiology
Standard radiological studies are indicated in cases of congenital hypothyroidism to confirm the presence of skeletal malformations. The radiographic studies of the appendicular skeleton can show: a delay in epiphyseal ossification and epiphyseal dysgenesis (irregular shape, presence of fragments, striations of the epiphyseal centres) visible particularly at the proximal end of the tibia and femoral and humeral condyles; a reduced length of all the long bones; carpal and/or metatarsal valgism; thinning of the cortex of the radius and ulna associated with rarefaction of the bone marrow. Axial skeletal images show a reduction in the dimensions of the vertebral bodies, which can have serrated ventral margins and short, broad cranial bones nor correctly attached to each other.
Scintigraphy
Scintigraphy is a particularly useful investigation for determining the underlying cause of the congenital form of hypothyroidism. In subjects with thyroid agenesis there is minimal uptake of pertechnetate in the usual site of the thyroid whereas in the forms caused by dyshormonogenesis there are defects in deiodination, the thyroid lobes are large and the uptake of pertechnetate is normal.
SPECIFIC DIAGNOSTIC TESTS
The definitive diagnosis of congenital hypothyroidism is based on the TSH stimulation test using recombinant human hormone. In animals with goitre, it is sufficient to measure total T4 to confirm the diagnosis, since the enlarged thyroid is itself an indicator of increased secretion of TSH.
TREATMENT
With the exception of the form of hypothyroidism caused by a lack of iodine, the treatment of all other forms of juvenile hypothyroidism is oral replacement with L-levothyroxine at a starting dose of 10 μg/kg b.i.d..
PROGNOSIS
The earlier the diagnosis is made and the treatment started, the less severe the permanent defects will be in the adult animal, especially with regards to skeletal development. A progressive improvement in the metabolic and dermatological clinical signs will be seen following hormone replacement therapy, while mild signs of mental retardation may persist.
Suggested readings
- Feldman E.C., Nelson R.W.: Hypothyroidism. In:Canine and Feline Endocrinology and Reproduction, III edizione Saunders, pg. 87-151, 2004.
- Rijnberk A., Kooistra H.S.: Thyroids. In: Rijnberk A., Kooistra H.S.: Clinical Endocrinolgy of Dogs and Cats, II edizione Schlutesche, pg. 55-91, 2010.
- Taeymans O. et al.: Thyroid imaging in the dog: current status and future directions. J Vet Intern Med,21, (4), 2007.