TSH stimulation test

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The TSH stimulation test is an adjunct test for determining hyperthyroidism in cats.

Exogenous TSH is a potent stimulator of thyroid hormone secretion. However, serum total T4 concentrations show little or no increase after exogenous bovine TSH administration in hyperthyroid cats[1]. Presumable this is because the thyroid gland of affected cats secretes thyroid hormones independently of TSH control, or that T4 already is being produced at a near maximal rate with limited reserve capacity[2].

Hyperthyroid cats with serum total T4 concentrations that are equivocally high tend to show results that are indistinguishable from healthy cats [3]. In addition, bovine TSH is no longer available for parenteral administration. Recombinant human TSH has been evaluated in hyperthyroid cats and has a significant cost implication.

Overall, because of its poor diagnostic performance and high cost, the TSH stimulation test is not recommended for hyperthyroidism.

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
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References

  1. ↑ Mooney, CT, Thoday, KL & Doxey, D (1996) Serum thyroxine and triiodothyronine responses of hyperthyroid cats to thyrotropin. Am J Vet Res 57:987-991
  2. ↑ August, JR (2006) Consultations in feline internal medicine. Vol 5. Elsevier Saunders, USA
  3. ↑ Mooney, CT, Thoday, KL & Dozey, D (1996) Serum thyroxine and triiodothyronine responses of hyperthyroid cats to thyrotropin. Am J Vet Res 57:987-991
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