Endocrinology - Hypothyroidism Flashcards
What does TBG stand for?
thyroid hormone binding globulin
What does TSH stand for?
thyroid stimulating hormone - thyrotropin
What does TRH stand for?
Thyrotropin releasing hormone
What does T4 stand for?
Thyroxine
What does TgAA stand for?
Thyroglobulin antibodies
What does T4AA stand for?
T4 autoantibodies
What does NTI stand for?
non-thyroidal illness
What does ED stand for? (in relation to the thyroid not male pee pee issues)
equilibirum dialysis
What does RIA stand for?
radioimmunoassay
The thyroid hormone has a lot of functions. It is important to keep in mind that ___ ______ ______ escapes the adverse effects of excess or insufficiency.
No organ system
What is the major secreted thyroid hormone?
T4
What is the major biologically active thyroid hormone?
T3
Is T3 or T4 more potent?
T3
What form of hormone, bound or unbound, is biologically active and able to mediate regulatory feedback?
The unbound (free) form
What are the 2 major categories of primary hypothyroidism?
Lymphocytic thyroiditis or idiopathic thyroid degeneration
What is lymphocytic thyroiditis?
Lymphocytic infiltration with progressive destruction of thyroid follicles
How can inflammation in cases of lymphocytic thyroiditis be measureD?
In the serum by measuring antibodies to thyroid components - usually thyroglobulin
True or False: You will see clinical signs well before extensive damage by lymphocytic thyroiditis occurs.
False - extensive destruction occurs before clinical signs of hypothyroidism develops
What is idiopathic thyroid degeneration?
Loss of thyroid parenchyma and then replaced by adipose and fibrous tissue
What breeds are over represented by hypothyroidism?
Doberman, golden retriever, labrador retriever, and cocker spaniel
When do clinical signs due to hypothyroidism typically develop?
in middle aged animals
What general clinical signs are associated with hypothyroidism?
Decreased metabolic rate, skin/haircoat changes (most common), neurologic, ocular, and myxedema coma
What clinical signs are associated with decreased metabolic rate?
lethargy, dullness, weight gain without polyphagia
What skin/haircoat changes are associated with hypothyroidism?
Alopecia, hyperpigmentation, and pyoderma
What neurologic clinical signs are associated with hypothyroidism?
Peripheral neuropathy, central vestibular disease, subclinical myopathy, and less commonly (seizures, laryngeal paralysis, megaesophagus, MG, and others)
What ocular clinical signs are associated with hypothyroidism?
corneal lipid
What is a myxedema coma?
life-threatening hypothermia, bradyarrhythmias, and rapid deterioration in the mental state
What clinical pathology is associated with hypothyroidism?
Normocytic, normochromic, non-regenerative anemia (30% of dogs), hypercholesterolemia (75% of dogs), hypertriglyceridemia, and a normal UA
What thyroid screening test is used to rule out hypothyroidism?
Total thyroxine (TT4)
What forms of T4 does TT4 screen for?
bound and unbound
True or False: A TT4 can diagnose hypothyroidism.
False - you should never use a low T4 in isolation to diagnose hypothyroidism
Why is as TT4 screen helpful in ruling out hypothyroidism?
Because very few true hypothyroid dogs will have a normal T4
What can cause a low TT4 in euthyroid dogs?
Breed variation, age, random fluctuations/diurnal rhythm, drugs, and non-thyroidal illness
What drugs can cause a decrease in TT4 and fT4, an increase in TSH, and clinical signs of hypothyroidism?
Sulfonamides
What drugs can cause a decrease in TT4, a decrease to normal fT4, a normal TSH, and no clinical signs?
glucocorticoids
What drug can cause a decreae in TT4 and fT4, an increase in TSH (but within the reference range), and no clinical signs?
Phenobarbital
What is non-thyroidal illness/sick euthyroid syndrome?
A syndrome where there are low thyroid levels in a patient with a fully functioning thyroid
What can cause sick euthyroid syndrome?
systemic illness, surgery, trauma, and malnutrition
How does sick euthyroid syndrome effect thyroid diagnostics?
It commonly decreases TT4, decreases fT4, and rarely increases TSH
What is the ‘amazing-perfect’ diagnosis for euthyroid sick syndrome?
low TT4 and fT4 with a high TSH
What should you do if a patient has a concurrent illness but you suspect a thyroid problem?
Wait until the concurrent illness has resolved
What is the use for fT4 screening test?
It is used in place of TT4 as a screening test or as a follow-up to a low TT4 if there is a non-thyroidal illness
What causes an elevated TSH level?
Lack of negative feedback in primary hypothyroidism
What is the use for a TSH test?
It is used in combination with TT4 and/or fT4 to support a diagnosis of primary hypothyroidism
True or False: Elevated TSH is highly specific for hypothyroidism
TRUE
What thyroid screening test is not helpful to test for hypothyroidism in dogs and why?
Total T3 because a large proportion of TT3 is not made in the thyroid gland, and it is not the predominate circulating hormone - 90% of hypothyroid dogs have a normal TT3
How can thyroiditis be diagnosed?
Via antibodies (serum thyroglobulin autoantibodies) in circulation during the inflammatory phase of disease
What is the thyroiditis antibody that circulates in dogs?
anti-thyroglobulin
How is the serum thyroglobulin autoantibodies test used?
To add support to suspicion of hypothyroidism when TT4/FT4+TSH is equivocal
What is the serum T3 and T4 autoantibody test used to do?
Clarify unexpected T4 or T3 results - it also supports diagnostic lymphocytic thyroiditis
When should you consider treatment trials?
When you have low TT4 and a normal TSH, normal TT4 and positive T4AA, and/or normal TT4 and high TSH with clinical suspician
How is hypothyroidism treated?
Levothyroxine sodium - 0.02 mg/kg BID
When is it critical to monitor treatment for hypothyroidism?
After 6-8 weeks of supplementation
With appropriate treatment, all clinical signs of hypothyroidism are ______.
reversible
What is important to inform your clients about the dermatologic abnormalities associated with hypothyroidism and treatment?
they can get worse before they get better
When monitoring response to treatment, what do you want to measure and when?
Serum T4 - 4-8 weeks after initiating therapy, when signs of thyrotoxicosis develop or when there has been minimal response to therapy
T4 and TSH - 4-6 hours after administration of supplementation if autoantibodies are present, you cannot monitor with T4 and need to use fT4ED
Where should the post-dose T4 level be in response to treatment?
in the upper half or above the reference range
Where should the TSH levels be in response to treatment?
in the reference range
The dose of thyroid hormone supplementation should be reduced if the post-pill T4 is greater than what value?
6 micrograms/dl
What is the life expectancy of a dog with hypothyroidism?
it should be normal with normal treatment
