Hypothyroidism

[Penel CIMDA moderator]

Hypothyroidism – An underactive thyroid gland

'More than 95% of cases of canine hypothyroidism are believed to be due to acquired primary hypothyroidism. Destruction of the thyroid gland can result from lymphocytic thyroiditis, idiopathic thyroid atrophy or rarely neoplastic invasion.'  Canine Medicine and Therapeutics by Neil Gorman.

Hypothyroidism or under-active thyroid glands, is the most common autoimmune disease in the dog and is probably evident in most breeds, although the incidence appears to be higher in some breeds than others. This disease is usually a slow process resulting in the destruction of the thyroid glands by antibodies directed against the thyroid (lymphocytic thyroiditis).  As the body only needs less than 20% thyroid production to function, clinical signs of this disease often develop over a lengthy period of time and are so gradual that the owner is not always aware that the dog is slowing down.  The body will cope very well with diminishing thyroid glands, but when the glands have been approximately 80% destroyed then clinical signs become more obvious. 

The thyroid is essential to life; it circulates thyroid hormones to all parts of the body. If replacement therapy is not given to a hypothyroid dog it will die.  Usually the clinical signs show well before this time and a thyroid blood test should confirm a diagnosis. Thyroid disease is not always simple to diagnose but usually a low TT4 (total thyroid hormone) confirmed by a low FreeT4 (thyroid hormone measurement without protein) and a high TSH (Thyroid Stimulating Hormone) is considered to be diagnostic of hypothyroidism.  (In addition, tests to assess thyroid dysfunction may include: TT3, FT3, autoantibody, TSH and TRH response tests).

Measuring TT4 alone is not enough to make a definitive diagnosis as the dog could be suffering from a non-thyroidal illness (NTI), that is an illness affecting the amount of thyroid hormone within the circulation, rather than a primary autoimmune destruction of the glands.  Non-thyroidal illness should be suspected if the dog has a low TT4 and a low TSH. Note: Several drugs, including prednisolone, will lower the circulating TT4 levels and this should be taken into account when evaluating the results. 

Note: Hypothyroidism is both over and under diagnosed.

Some clinical signs of hypothyroidism are:
Lethargy, mental dullness, unwillingness to exercise,  stiffness in gait – limping, weight gain (obesity), dry scaly skin or greasy skin, excessive pigmentation (hyperpigmentation),  skin lesions, on-going ear problems, coat texture and colour changes, loss of hair particularly on the tail and bilateral, symmetrical  hair loss, signs of premature ageing, irregular seasons - poor infertility and libido, depression (tragic expression), irritability or aggression, intolerance to cold, seizures, voice change (pitch of bark),  muscle weakness, megaoesophagus (weakness of muscles in the throat causing difficulty in swallowing).

On examination your vet may also find that a hypothyroid dog has a slow heart beat (bradycardia), eye abnormalities such as corneal lipid deposits or ulceration: dysfunction of the central nervous system (CNS), such as tilting of head, circling, unsteady gait (ataxia):  Blood analysis may also reveal anaemia and high cholesterol.

The prognosis for dogs with hypothyroidism is excellent, although life-long daily treatment with hormone replacement, Levothyroxine, is required. Giving half the daily hormone replacement dose every twelve hours provides a more balanced level of thyroid hormone throughout a 24 hour period and avoids peaks and troughs. (BSAVA Small Animal Endocrinology) 

Note: Sometimes dogs with hypothyroidism are prone to other autoimmune diseases.

Note:  Levothyroxine is better absorbed on an empty stomach - Canine Medicine & Therapeutics by Neil Gorman