Monitoring Treatment for Hypothyroidism

IDEXX Veterinary Services
November 2000

Monitoring thyroid supplementation

In dogs receiving thyroid supplementation: The post pill T4 should be assessed 2-4 weeks after starting therapy. (some authors recommend a pre pill (trough) and a post pill (peak) measurement).
The peak blood level should occur between 4-6 hours post pill. Thus, we recommend drawing blood for post pill testing at 4-6 hours post pill.  If the dose is adequate, the result should be in the normal range to increased (beyond the normal range); and there should be a good clinical response.  If there are has been good clinical response with a low T4 result, continue with the same dose and dosing schedule as the patient is currently receiving.  If clinical response is not satisfactory and T4 concentrations are low, increase the dose.  If the clinical response is not satisfactory and T4 concentrations are optimal or increased, reassess the diagnosis.

Below is a table to follow when measuring a pre and post T4 level.
 Frequency of administration
 normal      normal    no change
 normal  increased    no change unless signs of thyrotoxicosis or  T4 >7.5 g/dl
 low  normal to increased  s.i.d.  increase to b.i.d.
 low  normal to increased  b.i.d.  increase dosage or consider t.i.d.
 low  low    increase dosage, consider intestinal malabsorption, antithyroid hormone antibodies (antiT4 and antiT3 antibody). Use synthetic T3

There are a variety of factors which influence thyroid testing including aging, non-thyroidal illness, diet, drugs, individual variability in absorption and bioavailability of the thyroid supplement being given.

Monitoring cats on Methimazole (Tapazole)

The patient should have been on therapy for 3-4 weeks. Obtain blood for T4 testing 4-6 hours after the most recent pill.  If the dose is effective, the T4 should be in the reference range.  If the T4 is below the reference range but there are no clinical signs of hypothyroidism, maintain the current dose and dosing schedule.
It is also recommended that CBC, serum chemistry and urinalysis be performed prior to initiation of therapy.  If side effects are to develop, they will usually be detected in the first 4-8 weeks of therapy. Retesting these parameters every 2-4 weeks for the initial 8-12 weeks is recommended.

Refsal, K.R., Nachreiner, R.F., Laboratory Monitoring of Thyroid Supplementation; Proceedings from International Symposium on Canine Hypothyroidism p. 68-69.
Feldman and Nelson; Canine and Feline Endocrinology and Reproduction, 2nd edition, W.B. Saunders, 1996.