Thyroid - Ex 4 Flashcards
T3 and T4 - which is active form? which is more common in blood?
T3 is active form, responsible for negative feedback mechanism to pituitary and hypothalamus
T4 is major secretory product of the normal thyroid gland and is found in higher amounts in bloodstream
Serum T3 & T4
Free (active) and protein-bound (inactive)
Albumin is major protein that binds
Thyroid Hormone Functions
- inc basal metabolic rate
- inc O2 consumption
- inc HR
- inc catabolism of fat and muscle
- inc protein synthesis
- inc erythropoiesis (may affect CBC results)
- inc sensitivity to catecholamines (may affect CBC results)
- Critical for development of differentiation of cells
Canine Hypothyroidism
Most COMMON endocrinopathy in dogs!!
- middle aged dogs
- mid to large sized breeds
Canine Hypothyroidism clinical signs
Dermatologic: alopecia, pyoderma, otitis, etc.
Metabolic: lethargy, inactivity, weight gain
Neuromuscular: weakness, ataxia
Canine Hypothyroidism - Lab Abnormalities
- Hypercholesterolemia (>75% of dogs)
* Mild, non regenerative anemia –> BM can get affected; thyroid hormones are needed for proliferating cells
Pathogenesis of Hypothyroidism
- Congenital - abnormal thyroid development (RARE)
- Acquired:
- Primary: thyroid gland doesn’t produce hormone
- -95% of cases
- Secondary: Pit gland doesn’t produce TSH
- Tertiary: Hypothalamus doesn’t produce TRH
Thyroid Tests - Total T4
- Serum or plasma
- Normal thyroid function = frequently low T4 –> if low you can’t rule out hypothyroid
- Good for excluding hypothyroidism (if normal)
Thyroid Tests - Free T4
Most accurate in canine testing
If a patient is hypothyroid - what would you expect?
T4 to be low
T3 to be low
TSH to be high (primary acquired hypothyroid patient)
Thyroid Tests - Total T3 & free T3
Not recommended as sole test
Often within normal limits in hypothyroid dogs
Thyroid Tests - cTSH
Not good as single test
Hypothyroid dogs can have normal cTSH
&
Euthyroid dogs can have elevated
Whats the best combo of tests to dx hypothyroid in dogs?
fT4 & cTSH
TSH should be high and T4 should be low –> in true hypothyroid patient
More thyroid tests - TSH stimulation
TSH stimulation –> give dog TSH –> T4 production should increase in healthy dogs; in hypothyroid animals T4 remains low
Now uncommonly used
Equine Hypothyroidism
More likely to occur in foals –> rare in adults
Equine Hypothyroidism - Pathogenesis
Foals:
- Dietary iodine deficiency
- ingestion of goitrogen by mare
Adults:
- neoplasia
Equine Hypothyroidism - Dx
tT4, fT4, tT3, and fT3 –> same as dog
TSH –> different; we can get cTSH but hard to get eTSH
Feline Hypothyroidism
RARE condition
Most commonly iatrogenic –> OD of antithyroid drug or bilateral thyroidectomy
Hyperthyroidism - Dogs
Dogs:
RARE
May be due to functional thyroid tumor
- most thyroid tumors are nonfunctional (don’t secrete T3 or T4)
Hyperthyroidism - Horses
Horses:
RARE
May be due to functional thyroid tumor
- most thyroid tumors are nonfunctional (don’t secrete T3 or T4)
Exposure to excess iodine
- drugs, shampoos, leg paints, expectorants
Feline Hyperthyroidism
Most COMMON endocrinopathy in cats
> 8 years of age (mean is 13 years)
No breed or gender predilection
Feline Hyperthyroidism - Clinical signs
weight loss polyphagia PU/PD inc activity restlessness GI signs (d/v) Skin changes Res signs
Basically…. “SICK CAT”
Feline Hyperthydoiism - Lab Abnormalities
- Inc liver enzymes (ALT, ALP)
- >90% of cats - Erythrocytosis
- Excitement leukograms (neutrophilic, lymphocytosis)
- Inc BUN
- creatinine WNL (but usually have concurrent kidney dz so this will not be the case)
- due to inc protein catabolism - Dec cholesterol
- Falsely dec fructosamine (can mask DM)
Feline Hyperthyroidism - Pathogenesis
- Excess production of T4 & T3 by the thyroid gland
- almost always primary thyroid dz
- benign proliferations much more common
- malignant tumors uncommon
Feline Hyperthyroidism - Dx tT4
Total T4
- if high –> dx for hyperthyroidism
- if normal to high normal –> additional tests needed, or repeat test later
Feline Hyperthyroidism - Dx fT4
If normal, helps rule out hyperthyroidism
If elevated:
- high normal tT4 w/elevated fT4 = hyperthyroidism
- low to low normal tT4 w/ elevated fT4 = euthyroid
Feline Hyperthyroidism - Dx T3 Suppression Test
Not commonly used
Give 7 doses of synthetic T3 q8hr
Collect post-administration tT4
Interpretation:
- Normal: 50% reduction in tT4
- Abnormal: minimal to no reduction in tT4
If increased tT4…
felines
HYPERTHYROID - cats
