21 – Thyroid Flashcards
Hypothalamic-pituitary axis
- T3: most active form
o rT3 is inactive - most T3 and T4 are protein bound
o only “free” hormone can enter cells to exert biological action
o free T4 is a better assessment of thyroid function than total T4 - T3 is rarely measured
o Reflects tissue activity rather than thyroid gland function
Total T4 for thyroid testing
- Includes both protein-bound and free T4
- Most common initial screening test
- Inexpensive, widely available, typically measured on serum
- May be affect by non-thyroid factors
What are some non-thyroid factors that may affect total T4?
- Lower in elderly dogs and certain breeds (greyhounds, sighthounds)
- Subnormal at random times throughout the day
- Decreased by certain drugs (steroids, phenobarbital, NSAIDs, sulfa antibiotics)
- **Decreased by concurrent non-thyroidal illness
Nonthyroidal illness syndrome (NTIS)
- Also called: ‘euthyroid sick syndrome”
- Suppression of thyroid hormone concentration as a result of acute or chronic illness
- Most often see decreased total T4
o Free T4 less likely to be affected, but may be decreased with substantial illness
o 8-10% of dogs with NTIS will have increased TSH
Thyroid stimulating hormone (TSH) for thyroid testing
- May help to differentiate primary HYPOTHYROIDISM from other causes (ex. NTIS)
o Expect high TSH with hypothyroidism
o Normal TSH with NTIS
o Cannot accurately measure low TSH
What are some limitations to TSH for thyroid testing?
- 20-30% hypothyroid dogs have TSH within normal range
- 8-10% of NTIS dogs have increased TSH
- Increased TSH occasionally seen in euthyroid dogs treated with certain drugs (steroids, sulpha drugs, etc.)
- *imperfect test
Free T4 for thyroid testing
- Metabolically active fraction of T4
- Often measured in dogs with non-diagnostic total T4 test results
- “equilibrium dialysis” is the GOLD standard method (most expensive)
o No interference by T3AA or T4AA
o RIA, CLIA methods less sensitive, can see interference - *Free T4 is less affected by NTIS and drugs
- Reference range for free T4 may be lower in some breeds (ex. greyhounds)
What are the tests for lymphocytic thyroiditis?
- Predominant antibody is against thyroglobulin (TgAA)
- Thyroid hormone autoantibodies (T3AA, T4AA)
Predominant antibody is against thyroglobulin (TgAA): test for lymphocytic thyroiditis
- Positive result is suggestive of thyroid pathology
- *Provides no assessment of thyroid function
o **do NOT use to diagnose hypothyroidism - Sometimes used as a screening test by dog breeders
Thyroid hormone autoantibodies (T3AA, T4AA): test for lymphocytic thyroiditis
- Occur in 5-8% of cases of canine hypothyroidism
- May falsely increase measured total T4 and free T4 (by RIA or CLIA)
o Testing for T3AA/T4AA indicated in dogs with unexpected or unusual results
o Only free T4 equilibrium dialysis is NOT affected - Provides NO assessment of thyroid gland function
Canine hypothyroidism
- > 95% of cases result from destruction of the thyroid gland (primary hypothyroidism)
- Most common large breed dogs, 4-10 years old
- One of the most OVER-DIAGNOSED diseases in dogs
o Only test when you have a strong clinical suspicion for hypothyroidism
What are the common lab findings with canine hypothyroidism?
- Normocytic, normochromic, nonregenerative anemia
o 40-50% cases
o Lack of TH stimulation on hematopoietic cells and decreased oxygen consumption in tissues - Fasting hypercholesterolemia
o ~80% of cases
o Decrease degradation of lipids and clearance of lipids
How do you diagnose canine hypothyroidism?
- Start with panel of total T4 and TSH
o Low total T4 and high TSH=consistent with hypothyroidism
What do you do if you get discordant results with trying to diagnosis canine hypothyroidism?
- Monitor and retest at a later date (especially if NTIS possible)
- Evaluate a free T4 (and TgAA, T3AA, T4AA if warranted)
- Therapeutic trial of thyroid hormone supplementation
Therapeutic trail of thyroid hormone supplementation (canine hypothyroidism diagnosis)
- Every attempt should be made to exclude non-thyroid illness
- Objective criteria should be used to assess response to treatment
- Ideally would withdraw supplementation to confirm that clinical signs return (no one really does that though
Monitoring therapy with levothyroxine (canine hypothyroidism)
- Recheck total T4 levels
o 6-8 weeks after starting treatment
o 1-4 weeks after adjusting the dose
o OR if signs of thyrotoxicosis develop - Collect blood sample 4-6 hours post-pill
- May take 2-3 months to see improvement in dermatological signs and obesity
- Neurological deficits tend to improve rapidly, but full resolution may take 2-3 months
What do you do if therapy is unsuccessful? (canine hypothyroidism)
- Check total T4 to make sure dosage is correct
- Look for non-thyroidal illness
o *INCORRECT diagnosis is MOST common cause of treatment failure - Retest thyroid parameters minimum 4 weeks (6-8 weeks preferred) after supplementation discontinued
Feline hyperthyroidism
- Most often caused by thyroid gland hyperplasia OR benign thyroid adenoma
o Thyroid carcinoma rare in cats (1-2% of cases)
o Functional=produce thyroid hormones
What are the clinical signs of feline hyperthyroidism?
- Weight loss (90%) and polyphagia
- Polydipsia/polyuria
- Behaviour changes
- GI signs
- Unkempt hair coat
- Palpable cervical mass: NOT pathognomonic
- Cardiac disturbances: tachycardia, arrhythmias, CMO
What are the common laboratory findings of feline hyperthyroidism?
What are the common laboratory findings of feline hyperthyroidism?
- Mild to moderate elevation of liver enzymes (ALP, ALT)
o May return to normal with resolution of hyperthyroid state
- Mild erythrocytosis
o 40-60% of cases
o Excessive thyroid hormone stimulates bone marrow precursors and EPO
- Cats are often geriatric=many will have concurrent disease
Feline hyperthyroidism and renal failure
- ~30% will develop chronic renal failure after treatment of hyperthyroidism
o hyperT4 can mask CRF by increased GFT
o if untreated, hyperT4 related systemic hypertension can lead to intraglomerular hypertension and glomerulosclerosis=develop/worse chronic renal failure
How do you diagnosis feline hyperthyroidism?
- Total serum T4
o Elevated in 90% hyperthyroid cats
What do you do if total T4 is normal, but you still strongly suspect hyperthyroidism (feline)?
- Recheck total T4 at a later date (total T4 has daily fluctuations)
- Measure free T4
- Rule out non-thyroidal illness
o NTIS can lower T4 into the normal range
o Stabilize/resolve NTIS before retesting
What are some treatment options for feline hyperthyroidism?
- Methimazole: oral/topical
- Diet
- Surgery
- Radioactive iodine therapy
