Cutaneous manifestations of internal disease exist in both human and veterinary medicine. Many endocrine skin diseases start with bilaterally symmetrical alopecia as the first clinical and presenting sign.
The Master gland of the endocrine system is the pituitary gland. The anterior portion of the pituitary is the most important part of the gland concerning dermatologic diseases. Anterior Pituitary Hormones include TSH, ACTH, FSH, LH, B-lipoprotein, Prolactin and GH.
Hypothyroidism is a common multisystemic disease that is present in dogs and even occasionally in cats. It occurs predominately in the middle aged purebred dogs including the Golden retriever, Doberman Pinscher, Irish setter, Boxer, Miniature schnauzer, Dachshund and Cocker spaniel.
Hypothyroidism is the most common endocrine dermatosis, but also the most commonly overdiagnosed. Primary atrophy and destruction of the thyroid gland is the most common cause of hypothyroidism. Lymphocytic thyroiditis is an immune mediated destruction of the glandular tissue. Circulating antithyroglobulin antibodies are present in many cases. Idiopathic thyroid gland atrophy can also be seen. In these cases the thyroid follicles are replaced with adipose tissue without inflammation. Secondary and tertiary hypothyroidism is very rare.
- In the body, the thyroid hormone T4 is converted to T3, the major active hormone and reverse T3, which is inactive.
- In cases of hypothyroidism, epidermal atrophy and abnormal keratinization is present. There is also decreased protein synthesis, abnormal lipogenesis and decreased sterol synthesis of keratinocytes. There are decreased anagen hairs, increased Glyscosaminoglycans in the dermis. Recurrent superficial bacterial pyoderma is common. Symmetrical alopecia, dull, dry pelage that fails to grow after clipping, variable hyperpigmentation, cool, dry skin that easily bruises and
- Seborrhea are many of the cutaneous signs.
- Other clinical signs include weight gain, lethargy, cold intolerance, weakness, peripheral neuropathy, bradycardia, impaired myocardial contractility, decreased QRS amplitude, anestrus, lack of libido, corneal lipid deposits, and KCS.
- Screening laboratory tests show normocytic, normochromic anemia, hypercholesterolemia, hypertriglyceridemia (less common).
- Serum T4, fT4, T3, fT3, rT3, antithyroglobulin antibodies and endogenous TSH are all available. TSH response and TRH stimulation are no longer available for private veterinary practitioners.
- Low T4 levels are seen in hypothyroidism, sick euthyroid syndrome GCC, anticonvulsants, potentiated sulfas drugs and androgens/estrogens.
- Preliminary data shows that running fT4 and tT4 may increase accuracy by 10-15%. however measuring fT4 alone has no greater value than tT4 by itself.
- Endogenous TSH levels should be elevated in primary hypothyroidism and low in sick euthyroid syndrome. Antithyroid antibodies include measurement of antithyroglobulin antibodies and antibodies to T3
- Treatment of Hypothyroidism is L-T4 at a dose of 0.01-0.02 mg/kg bid
- Large dogs may be dosed at 0.5 mg/m2 bid. Post-pill T4 should be monitored
- L-T3 therapies and L-T3/L-T4 combination therapies are not recommended