T3-suppression test

From Cat

The T3-suppression test is an alternative test to the Thyrotropin-releasing hormone (TRH) stimulation test for testing feline hyperthyroidism.

Inhibition of pituitary thyroid-stimulating hormone (TSH) secretion by high circulating concentrations of thyroid hormone is a characteristic feature of normal pituitary-thyroid regulation[1]. Therefore, administration of T3 to normal cats leads to a decrease in TSH secretion, which in turn causes diminished T4 secretion. In contrast, when thyroidal function is autonomous (i.e. independent of TSH secretion), administration of thyroid hormone has little or no effect on serum T4 concentrations, because TSH secretion already has been suppressed chronically.. This is invariably true with clinical hyperthyroidism in cats[2].

When the T3 suppression test is performed in normal cats, a marked falls occurs in serum T4 concentrations after exogenous T3 administration. In contrast, when the test is performed in hyperthyroidism cats, even in cats with only slightly high or high-normal resting serum T4 concentrations, minimal, if any, suppression of serum T4 concentration is seen.

To perform the T3 suppression test in cats, a serum (clotted blood) sample is drawn for determination of basal serum concentrations of total T4 and T3. This sample is refrigerated. Owners are instructed to give T3 orally (Liothyronine; 'Cytomel') beginning the following morning at a dosage of 25µg q8hr for 2 days. On the morning of the third day, a seventh 25-µg dose of liothyronine is given and the cat returned to the veterinary clinic within 2 to 4 hrs for serum T4 and T3 determinations. Both the basal (day 1) and post-liothyronine serum samples should be submitted to the laboratory together to eliminate the effect of interassay variation in hormone concentrations.

Commonly used protocols for dynamic thyroid function tests in cats (August, 2006)


T3-suppression test Thyrotropin-releasing hormone (TRH) stimulation test TSH stimulation test TSH stimulation test
Drug Liothyronine (Cytomel) TRH Bovine TSH Human TSH
Dose 25 µg q 8hr for 7 doses 0.1 µg/kg 0.5 IU/kg 0.025 - 0.20 mg/cat
Route Oral Intravenous Intravenous Intravenous
Sampling time Before and 2-4 hrs after last dose Before and at 4 hrs Before and at 6 hrs Before and at 6-8 hrs
Assay Total T4 and T3 Total T4 Total T4 Total T4
Interpretation:
Euthyroidism T4 <20 nmol/L with >50% suppression >60% increase in T4 >100% increase in T4 >100% increase in T4
Hyperthyroidism T4 >20 nmol/L with <3% suppression <50% rise in T4 Minimal to no increase Not determined


References

  1. Utiger, RD (1986) Tests of thyroregulatory mechanisms. In Ingbar, SH & Braverman, LE (Eds): The thyroid: a fundamental and clinical text. Lippincott, Philadelphia. pp:511-523
  2. August, JR (2006) Consultations in feline internal medicine. Vol 5. Elsevier Saunders, USA