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Hypothyroidism   525




            Hypothyroidism                                                                         Client Education
                                                                                                          Sheet
  VetBooks.ir                                                                                                         Diseases and   Disorders

                                                  change in bark [laryngeal paralysis]) are
            BASIC INFORMATION
                                                                                    ence of typical clinical signs in conjunction
                                                  uncommon.                         is not always straightforward, but the pres-
           Definition                           ○   Possible behavioral changes (e.g., aggression)  with a decreased serum T 4  or free T 4  after
           Clinical syndrome that occurs as a result of   •  Myxedema coma, characterized by profound   equilibrium dialysis (FT 4 ED) concentrations
           decreased circulating levels of serum thyroid   mental dullness, stupor, hypothermia,   and an increased serum TSH concentration
           hormones                             hypoventilation, bradycardia, and hypoten-  can be considered diagnostic.
                                                sion, has been reported rarely.   •  Diagnosis of iatrogenic hypothyroidism in
           Epidemiology                        •  Congenital  hypothyroidism:  dispropor-  cats is based on clinical signs combined with
           SPECIES, AGE, SEX                    tionate  dwarfism,  short  limbs,  persistent   low  serum  T 4   and  increased  serum  TSH
           •  Dogs:  middle-aged  at  onset  (2-9  years  of   puppy coat or alopecia/thin dull hair, broad   concentrations (measured with a canine
             age), no sex predisposition, rare in puppies   head, thick protruding tongue, mental    TSH assay).
             (i.e., congenital form)            dullness/retardation, delayed dental eruption,
           •  Cats: spontaneous and congenital hypothy-  constipation             Differential Diagnosis
             roidism rare; iatrogenic disease in middle-  •  Iatrogenic hypothyroidism in cats: can follow   •  Alopecia (p. 47)
             aged or older cats                 treatment for hyperthyroidism. Signs may be   •  Cutaneous cornification disorders (p. 214)
                                                absent or vague. If present, signs of lethargy,   •  Obesity/weight gain (p. 700)
           GENETICS, BREED PREDISPOSITION       weight gain, inappetence, constipation, and   •  Hypercholesterolemia (p. 1234)
           •  Dogs:  can  occur  in  any  breed,  including   dermatologic signs (especially seborrhea and
             mixed breeds                       ease of epilation) predominate.   Initial Database
           •  Reported  to be  more prevalent  in boxers,                         •  CBC, serum biochemical profile, urinalysis:
             dachshunds, Doberman  pinschers, golden   PHYSICAL EXAM FINDINGS       hypercholesterolemia is most common (must be
             retrievers, Great Danes, Irish setters, min-  •  Exam findings are similar to signs described   differentiated from normal postprandial hyper-
             iature schnauzers, poodles, and a number   by the owners (see above).  cholesterolemia [p. 1324]). A mild normocytic,
             of other breeds                   •  Additional  findings  (uncommon)  include   normochromic, nonregenerative anemia;
           •  Genetic risk loci have been identified in some   ocular  changes  (corneal  lipid  deposits   fasting hyperlipidemia/hypertriglyceridemia;
             breeds.                            most common), bradycardia, hypothermia,   mild increases in liver enzymes; and a mild
                                                constipation, facial paresis, vestibular signs,   increase in creatinine kinase may also be seen.
           ASSOCIATED DISORDERS                 and decreased spinal reflexes.    •  Basal serum total thyroxine concentration
           Can rarely occur as part of an autoimmune                                [T 4 ]
           polyglandular syndrome along with hypoad-  Etiology and Pathophysiology  ○   Includes free (<1%) and protein-bound
           renocorticism or diabetes mellitus  •  Primary hypothyroidism is usually due to   (>99%) T 4
                                                lymphocytic thyroiditis or idiopathic thyroid   ○   Measurement of a concentration below the
           Clinical Presentation                gland atrophy (uncommonly due to infiltra-  reference range in dogs has a sensitivity
           DISEASE FORMS/SUBTYPES               tive neoplasia).                      of 89%-100%, specificity of 75%-82%,
           •  Primary hypothyroidism (most common):   •  Secondary hypothyroidism (rare): decreased   and accuracy of 85%.
             lymphocytic infiltration or atrophy of the   TSH secretion leads to atrophy of the fol-  ○   If T 4  is in the middle or upper end of
             thyroid gland                      licular cells of the thyroid gland. Loss of TSH   the  reference  range,  hypothyroidism  is
           •  Congenital hypothyroidism: rare   secretion can occur as a result of a congenital   generally ruled out. An exception is the
           •  Secondary hypothyroidism (rare): inadequate   pituitary malformation or pituitary neoplasia.  presence  of  antithyroid  antibodies  that
             pituitary secretion of thyroid-stimulating   •  Tertiary hypothyroidism (decreased secretion   interfere with some assays; if clinical signs
             hormone (TSH)                      of thyrotropin-releasing hormone [TRH])   strongly suggest hypothyroidism and T 4
           •  Iatrogenic  hypothyroidism  resulting  from   has rarely been reported in dogs.  or FT 4ED is normal, advanced thyroid
             treatment of hyperthyroidism (cats)  •  Iatrogenic  hypothyroidism  can  occur  as   testing is recommended.
                                                a result of antithyroid medications (e.g.,   ○   T 4 that is low or at the low end of the
           HISTORY, CHIEF COMPLAINT             methimazole in cats), bilateral thyroidectomy,   reference range may be due to hypothy-
           •  Acquired disease: signs are often nonspecific   or radioiodine (I-131) therapy (p. 503).  roidism, concurrent illness, or medications
             and gradual in onset.             •  Clinical signs reflect the widespread metabolic   or may be normal (euthyroid). Be aware
             ○   Metabolic: weight gain, lethargy, mental   effects of thyroid hormones.  that certain breeds such as sighthounds,
               dullness, exercise intolerance/inactivity,   •  Surgery or anesthesia, a wide range of con-  Alaskan sled dogs, and Chinese Shar-peis
               and cold intolerance are common.  current illnesses, and various nonthyroidal   normally have significantly lower  T 4
             ○   Dermatologic: bilaterally symmetrical,   medications (e.g., glucocorticoids, phenobar-  concentrations  (approximately  one-half
               truncal  alopecia  or thin  hair, hyper-  bital, sulfonamides, carprofen, clomipramine)   of reference range of other breeds).
               keratosis, seborrhea, hyperpigmentation,   also decrease circulating thyroid hormone   •  Basal serum FT 4 ED
               otitis, “rat tail,” and pyoderma are often   concentrations; clinical hypothyroidism does   ○   Measures only free (i.e., metabolically
               present, with alopecia or thin hair most     not result from any with the exception of   available) T 4 fraction
               common.                          sulfonamides.                       ○   An assay based on equilibrium dialysis
             ○   Reproductive: persistent anestrus, decreased                         should be used; it is more time intensive
               libido, decreased survival of puppies born    DIAGNOSIS                and expensive than radioimmunoassay but
               to hypothyroid dams                                                    much more accurate.
             ○   Neuromuscular: weakness/exercise intoler-  Diagnostic Overview     ○   Measurement of a concentration below the
               ance sometimes seen; other signs (ataxia,   •  The diagnosis should be suspected in a young   reference range in dogs has a sensitivity
               seizures, facial drooping/paralysis,  head   to middle-aged dog presenting with one or   of 80%-98%, specificity of 93%-94%,
               tilt/circling [vestibular signs], and stridor/  more common clinical sign. Confirmation   accuracy of 95%.

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