Hypothyroidism Flashcards
1
Q
describe thyroid hormone regulation
A
- hypothalamus: secretes thyrotropin-releasing hormone (TRH)
-upregulated in response to decreased temperature
-downregulated/inhibited in response to severe/chronic stress - TSH stimulates the anterior pituitary gland to secrete thyroid stimulating hormone (TSH/thyrotropin)
- TSH stimulates the thyroid gland to secrete thyroxine (T4) and triiodothyronine (T3), which increase metabolism
-elevated T3 and T4 concentrations in the blood have negative feedback on the hypothalamus to decrease TRH secretion
-the thyroid gland produces a majority T4 and only a small amount of T3 but T3 is the active hormone!
-T4 is like a prohormone
2
Q
describe the functions of thyroid hormones (7)
A
- basal metabolic rate!! PRIMARY FUNCTION
- cardiovascular function
- renal function
- fetal growth (including skeletal development)
- CNS fetal development
- GI tract (increase motility)
- hematologic function (increase erythropoiesis)
3
Q
GENERALLY describe common thyroid diseases
A
dogs: primary hypothyroidism
cats: primary hyperthyroidism
but EITHER can occur in any species with a thyroid gland
both conditions have SLOW onset of clinical signs that may be present for months to years before diagnosis
4
Q
describe hypothyroidism
A
- much more common in dogs than cats
-dogs: acquired disease, adult onset
-cats: more likely iatrogenic (common) or congenital forms (uncommon) - disease when at least 80% of thyroid function is lost
-because thyroid has a large functional reserve - 95% of cases in dogs are primary
-problem is in the gland itself
-primary: lymphocytic thyroiditis (immune destruction) which leads to thyroid atrophy
-or idiopathic thyroid atrophy
-rarely: thyroid carcinoma, radiation-induced, congenital, or central hypothyroid
5
Q
describe the clinical presentation of hypothyroidism
A
- middle-age to older
-average at onset approx 6 years
-congenital signs noted <12 weeks - common breeds:
-golden and labrador retrievers
-doberman pinscher - genetic predisposition: several breeds
6
Q
describe clinical signs of hypothyroidism
A
- alopecia:
-endocrine alopecia: bilateral, truncal, loss of guard hairs, dry, lusterless, flakey coat, hyperpigmented skin, rat tail
-poor coat quality
-recurrent pyoderma/otitis - lethargy, exercise intolerance
-neuromuscular weakness - weight gain:
-often without change in appetite or diet - heat seeking behaviors:
-cold interolance - personality change, depression
- constipation:
-unusual in acquired disease, but common in congenital form
7
Q
describe clinical signs seen on physical exam of a hypothyroid dog
A
- overweight, obese
- alopecia, seborrhea
- mentally dull
- +/-
-hypothermia
-bradycardia: not as common now because we diagnose earlier due to screening
-myxedema: tragic facial expression due to accumulation of hyaluronic acid and chondroitin sulfate (glysominoglycan) in the dermis causing excessive tissue gel in interstitium
-myxedoma coma: altered/depressed mentation, hypoventilation, hypothermia, bradycardia (RARE and NOT a coma) - other possible manifestation:
-facial nerve paralysis
-vestibular paralysis
-polyneuropathy
-megaesophagus (regurgitation?)
-laryngeal paralysis
-lipid corneal dystrophy/lipemic uveitis
-decreased tear produciton
-decreased fertility/parturient problems
-gallbladder mucocele
8
Q
describe congenital hypothyroidism
A
rare disorder
- slow growth rate with disproportionate dwarfism
-short limbs
-broad head - lethargy, mental dullness
- constipation
- retained deciduous teeth, gingival overgrowth
- retained kitten hair coat
9
Q
describe clinicopathologic findings in hypothyroidism
A
CBC:
1. non-regenerative anemia
-mild, normocytic/normochromic
2. +/- thrombocytosis
biochem!!
1. hypercholesteremia!! MOST COMMONLY SEEN ABNORMALITY
-assess fasted
2. hypertriglyceridemia
3. +/- elevated ALP, ALT (ALP much more elevated than ALT, cholestatic pattern)
urinalysis:
1. not much there
2. +/- proteinuria from thyroiditis-associated glomerulonephritis (rare)
10
Q
describe hormone assessment of hypothyroidism
A
- hypothalamus, TRH:
-very increased! no negative feedback from thyroid hormone - pituitary gland, TSH:
-increase! no negative feedback from thyroid hormone - thyroid gland, T4:
-decreased!
11
Q
describe thyroid hormones in circulation
A
- 99% carried by blood proteins
-thyroxine-binding protein (TBP, not in cats)
-transteyretin (TTR)
-albumin, lipoproteins
-passive reservoir!! - in free form (very small fraction)
-metabolically active
12
Q
describe screening tests for hypothyroidism
A
- total thyroxine concentration (TT4)
-TT4 = protein bound T4 + free T4 (fT4)
-if >1.5-2.0 ug/dL, hypothyroidism is unlikely
-sighthounds have lower TT4 and fT4 reference intervals though (their TSH reference interval is normal though) - LOW TT4 does NOT MEAN HYPOTHYROIDISM
-hypothyroidism is the most commonly mis/overdiagnosed endocrine disorder in dogs!!! - screening test = can trust a negative test but cannot always trust a positive (ruled IN)
-if positive screening test for hypothyroid, MUST perform confirmatory thyroid testing!!
13
Q
describe definitive diagnosis of hypothyroidism
A
- thyroid panel
-TT4, fT4, TSH
-classic positive = LOW TT4 and fT4 with HIGH TSH
-caveat: TSH can be normal in 20-30% of hypothyroid dogs, but will be high in most cases and can precede clinical disease - MUST include complete evaluation of your patient (test because suspect or because routine?)
-definitively hypothyroid: low TT4 and fT4 with high TSH, clinical signs +/- clin path abnormalities present
-most likely hypothyroid: low TT4 and fT4 with normal TSH, clinical signs +/- clin path abnormalities
-2 out of 3 rule: if 2 of the 3 concentrations suggest hypothyroid AND you suspected it to start with, it probably is hypothyroid even if TSH isn’t elevated
14
Q
describe non-thyroidal illness syndrome/euthyroid sick syndrome
A
- thyroid hormone secretion decreases (appropriately) in illness
- physiologic adaptation to reduce metabolism that reflects disease severity
-so be CAREFUL with interpreting TT4 in sick patients! - can occur with:
-severe stress
-illness, co-morbidities (various organs)
-trauma
-following surgery - TT4 is the most sensitive of the hormones we measure routinely, so it drops first
-fT4 only drops below normal with moderate-severe illness
-TSH is likely normal until severe disease; can increase slightly/transiently during recovery
-issue: false positive for hypothyroidism! part of routine panels from some labs so when send off labs for sick animal and comes back with low TT4 can lead to mis/overdiagnosis of hypothyroid - challenging to accurately diagnose hypothyroidism in sick patients
-so do NOT measure TT4 in sick dogs!
-or manage underlying disease, wait until recovery, and retest before making diagnosis - euthyroid sick syndrome is a protective mechanism
-does NOT require thyroid supplementation, could worse catabolic state
-unlikely to have a classic positive result (low TT4/fT4, high TSH); thyroid panels are there to help you
15
Q
describe drug-induced thyroid suppression
A
- glucocorticoids, phenobarbital: lower T4 as a biochemical change with no clinical signs of hypothyroid, dose and duration dependent
-endogenous excess cortisol can have same effect, so diagnose and treat before diagnosing concurrent hypothyroid - sulfonamide antibiotics: can cause CLINICAL hypothyroid (low TT4 and fT4, high TSH); inhibit thyroid peroxidase, reducing thyroid hormone synthesis
16
Q
describe anti-thyroid antibodies
A
- primary hypothyroidism has an autoimmune etiology for most dogs (thyroiditis)
-auto-Ab against T3, T4, or thyroglobulin - antithyroid hormone antibodies can interfere with assays and be spuriously read as TT4, this falsely increases TT4
- so if you have/see:
-high concern for hypothyroidism byt TT4 is normal
-or a normal dog with elevated TT4
-RUN ft4 by equilibrium dialysis (+/- TSH) or measure antibody concentrations
17
Q
describe treatment for hypothyroid
A
- sodium levothyroxine (synthetic TT4)
-ideally on an empty stomach - wait at least 4=8 weeks to monitor therapy
-clinical improvements often noted quickly
-dermatologic signs may take time to resolve - monitor:
-ask clients about their pet’s health/eval skin and body weight
-biochem monitoring: TT4 concentration 4-6 hours following pill (red top serum tube), +/- TSH if elevated at diagnosis - goals:
-TT4 high-normal to slightly elevated
-target post-pill TT4 of approx 3-5 or 6 ug/dL at peak concentration (will be lower during rest of day)
-normal TSH
-change dose as needed, usually in increments of 0.05-0.1mcg
18
Q
what do you do if you misdiagnosed hypothyroid?
A
- stop supplementation and wait 4-8 weeks
-thyroid axis needs time to recover - monitor via:
-thyroid panel
-clinical signs
-chemistry panel
