Hypothyroidism is an endocrine (hormonal) disease where the body does not have adequate levels of the hormone thyroid. Thyroid helps regulate the metabolic function of almost every system in the body. Lack of this hormone is not life threatening but causes poor resistance to cold as well as mental and physical slowing. Clinical signs can vary in nature, usually come on gradually, and are often vague. The disease usually affects dogs in middle age. Bullmastiffs can become afflicted by 2-3 years.

Clinical Signs
Signs are directly related to slowed metabolism, such as mental dullness, lethargy, exercise intolerance, heat-seeking, and weight gain without an increased appetite.

The most common observable abnormalities are changes in the skin and coat. These changes can include hair loss, dryness, scaling, dullness, and seborrhea. Hair loss typically is not a result of itching but hair readily falls out and regrowth of is slow. The pattern of hair loss usually involves the sides and underside of the body, usually symmetrical, sparing the head and lower legs. Hair loss of the tail (rat tail) may also develop. Preferential loss of guard hairs can result in a “puppy coat.” Skin can become darkened, easily bruised, and have poor wound healing abilities. In extreme cases the skin can become thickened, predominantly in the face, resulting in a “tragic expression.” The immune system is impaired and recurrent pyoderma (skin infections) can occur, which may cause itching.

Hypothyroidism can be a cause for reproductive dysfunction. Bitches may have prolonged inter-estrous intervals, fail to cycle, have weak or silent cycles, have prolonged menstrual bleeding, inappropriate milk flow, abort, or give birth to weak, dying, or stillborn puppies. Dogs may lack libido, have shrunken testicles, low sperm count, or other infertility problems.

Severe hypothyroidism can cause cardiovascular signs. The heart beat can become weak and the rate slow. Changes on an ECG may be detected.

Ocular (eye) conditions are rare with this disease but may include ulceration of the cornea (the surface of the eyeball), fat deposits in the cornea, uveitis (inflammation of structures within the eyeball), keratoconjunctivitis sicca (KCS or dry eye), or glaucoma.

The neuromuscular system may be affected. Weakness, stiffness, reluctance to move, dragging feet resulting in excessive wear on the tops of the toenails, and muscle wasting may be associated with low thyroid.

Decreased motility and loss of segmental rhythm of the gastrointestinal tract impairs normal digestive and absorptive processes resulting in diarrhea or constipation.

More than 95% of cases are considered primary hypothyroidism, caused by destruction of the thyroid gland itself. This may be due to an immune-mediated disease, a degenerative disorder, or rarely a congenital (born that way) problem.

Less than 5% are secondary (pituitary) hypothyroidism. The pituitary gland in the brain normally secretes thyroid-stimulating hormone (TSH) which stimulates the thyroid gland. Impaired pituitary function may be caused by a tumor or congenital (documented in dwarf German Shepherds) disorder.

Extremely rare is tertiary (hypothalmic) hypothyroidism. The hypothalamus (located higher in the brain) releases thyrotropin-releasing hormone (TRH) which stimulates the pituitary gland to release TSH.

Another rare, and yet undocumented, cause may be poor converters., where there is a low conversion rate of one form of the thyroid hormone, T3, to the more active form, T4.

Unfortunately it is not always a simple task to diagnose hypothyroidism and it is one of the most over diagnosed diseases in dogs. Many factors will affect the level of thyroid in the blood, such as time of day, concurrent illness, heat cycle, late pregnancy, age (elevated in newborns), and breed (tends to be lower in large and giant breeds). I am unaware of any breed specific studies performed on Bullmastiffs. There are also different forms of the thyroid hormone that can be measured: T3, T4, protein bound, or free of protein.

Blood levels of T4 have more diagnostic value than T3, but even when this value is low it is not absolutely diagnostic. Normal values will vary depending on the laboratory used, but will be around 1.5-4 mcg/dl.

The TSH (thyroid stimulating hormone) stimulation test is currently the best method to confirm hypothyroidism. This test involves collecting blood before and 6 hours after administering TSH. If the level of T4 in the blood fails to rise by 1.5 mcg/dl, it is highly suggestive of hypothyroidism.

Daily supplementation with thyroid hormone is the treatment. Use of synthetic L-Thyroxine (L- T4), twice a day, is recommended. The typical dosage is .05-.1 mg per 10 pounds. With large dogs it is advised to dose by body surface area, which is more proportional to metabolic rate than body weight. This dosage is .5mg/m2. There are charts to do the conversions (i.e., 100 lb. = 1.27 m2, requiring .6mg; 125 lb. = 1.48 m2, requiring .7mg). Monitor for signs of hyperthyroidism in case of overdosing. This may include increased water consumption, increased urination, rapid heart rate, restlessness, or diarrhea. At 2-4 weeks the thyroid level should be rechecked to ensure correct dosing. If measured at peak time (4-8 hours after pill was given) the blood T4 should be at the high-normal range.

Canine and Feline Endocrinology and Reproduction by Feldman & Nelson
Saunders Manual of Small Animal Practice by Birchard & Sherding

This information is provided for educational purposes only and is not a substitute for medical advice. It is not to be used for diagnosing or treating a health problem, nor is it a substitute for professional care. If you suspect that your dog may have a health problem, please consult your veterinarian.