Monitoring/Control

Once started on Leventa®, the patient should be monitored every four weeks until a euthyroid state has been achieved. Serum TSH and peak T4 concentration should be evaluated four to six hours after the patient receives Leventa® in order to evaluate response to therapy.

While there may be some variations between laboratories, the generally acceptable range for serum T4 concentration is 1.0 μg per dl to 3.5 μg per dl, and the serum TSH concentration should be less than 0.6 ng per ml.

In a patient that is well regulated with Leventa®, the serum T4 concentration four to six hours post treatment should fall in the upper half of the reference range, or above. The acceptable upper limit for the T4 concentration is 6.0 μg per dl as long as the patient is not exhibiting signs of thyrotoxicosis.1 

Remember that if autoantibodies to T4 are present, we cannot rely on serum T4 measurement to evaluate response to therapy. Instead the recommendation is to measure free T4 using equilibrium dialysis.2   The presence of autoantibodies to T4 will not affect the response to treatment with levothyroxine sodium supplementation.

 

Adjusting the Dose if T4 Is Low
If the peak serum T4 value is below 1.5 μg per dl four weeks after initiating therapy, the dose of Leventa® should be increased in 10 μg per kg increments. Therefore, the dose of a patient receiving Leventa® at 20 μg per kg s.i.d. (0.1 mL/10 lbs) with a peak T4 concentration below 1.5 μg per dl, should be increased to 30 μg per kg of Leventa® s.i.d.  Peak serum T4 and TSH concentrations should be reevaluated two to four weeks after changing the dose with further adjustments made as necessary.

Remember that food can decrease the absorption of any levothyroxine sodium product, and therefore Leventa® should be administered directly into the mouth on an empty stomach. If it is necessary to administer Leventa® on the patient’s food, it is important to maintain a consistent means of administration, and in this case, to routinely give Leventa® with food. This way, the absorption will be consistent, and this consistency will be maintained when evaluating the serum T4 concentration post administration. Keep in mind that it may be necessary to increase the dose for a patient receiving Leventa® on the food, simply due to the decreased absorption.

Adjusting the Dose if T4 Is High
If the serum T4 concentration is above 6.0 μg per dl, it is recommended to decrease the dose of Leventa® in 10 μg per kg increments, or to discontinue use of Leventa® if signs of thyrotoxicosis are present. Therefore, for a patient not showing signs of thyrotoxicosis, the dosage should be decreased from 20 μg per kg s.i.d. (0.1 mL/10 lbs) to 10 μg per kg s.i.d. When serum T4 values are greater than 6.0 μg per dl, it may be necessary to consider a misdiagnosis of hypothyroidism (see Euthyroid Sick Syndrome) or impaired clearance of thyroid hormones, which may occur with hepatic or renal insufficiency.1

For a patient who is showing signs of thyrotoxicosis, Leventa® should be discontinued until the signs have resolved, which usually takes one to three days.1   Once signs of thyrotoxicosis have resolved, decrease the dose of Leventa® in 10 μg per kg increments. (See Thyrotoxicosis below.)

 

Since many hypothyroid patients will initially lose weight when starting on a levothyroxine sodium product, it may be necessary to decrease the dose of Leventa® based on the new body weight.  By monitoring the serum T4 and TSH concentrations four weeks after starting a patient on Leventa® (and after two to four weeks after any dose change), adjustments to the dose of Leventa® may be made accordingly.

Thyrotoxicosis
Thyrotoxicosis is uncommon in the management of hypothyroidism, but may occur if a patient receives an excessive amount of levothyroxine sodium or if a patient has impaired clearance of thyroid hormone.  Chronic oversupplementaion with levothyroxine sodium is generally more likely to cause thyrotoxicosis than an acute overdosage.3   Signs of thyrotoxicosis include hyperactivity, panting, restlessness, polyuria, polydypsia and weight loss in spite of a voracious appetite.  If thyrotoxicosis is suspected, treatment with Leventa® should be suspended until the signs of thyrotoxicosis resolve, which usually occurs within one to three days after discontinuing thyroid supplementation.  The dose of Leventa® should be adjusted accordingly and the serum T4 and TSH concentrations should be measured two to four weeks after the dose change.

 

Long-Term Maintenance
Once the patient has been shown to be well regulated on Leventa®, (based on clinical improvement and serum T4 and TSH concentrations), check the serum T4 and TSH concentrations every six months, four to six hours post administration of Leventa®.

 

Clinical Response to Leventa® Therapy
Clinically, most dogs begin to show improvement in their activity level within one week of starting thyroid hormone replacement therapy, however it may take several months for skin lesions, hyperpigmentation and alopecia to resolve.  Healthy weight loss will likely be observed over the next few months following initiation of therapy, as exercise naturally increases and improvements in fat metabolism occur. Improvements of neurologic signs are usually seen in the first one to two weeks of treatment, but may take up to three months to occur.1

References:
1. Nelson and Feldman. Canine and Feline Endocrinology and Reproduction. Third Edition. Hypothyroidism pp 86-142. Elsevier Science (USA). 2004.
2. Sara Schachter, Richard W. Nelson *, Catherine Scott-Moncrieff, Duncan C. Ferguson, Tracey Montgomery, Edward C. Feldman, Larry Neal, and Philip H. Kass   Comparison of Serum-Free Thyroxine Concentrations Determined by Standard Equilibrium Dialysis, Modified Equilibrium Dialysis, and 5 Radioimmunoassays in Dogs;  J Vet Intern Med 18[3]:259-264 May-Jun'04 Retrospective Study 28 Refs

3. Camille DeClementi Safrit, VMD;  Acute Thyroid Hormone Supplement Overdosage;  Vet Med 96[6]:424-430 Jun'01 Toxicology Brief 10 Refs