Utility of a canine TSH assay for diagnosis and monitoring of feline hyperthyroidism

Abstract

Objective This retrospective study was initiated to evaluate the utility of TSH measurements using a common canine TSH assay to diagnose and monitor feline hyperthyroidism after radioiodine or thyreostatic drug treatment.

Material and methods The electronic database of the University of Veterinary Medicine Vienna was searched for combined TSH and T4 measurements. 217 pairs of TSH and T4 from 136 cats with possible hyperthyroidism were assigned to group A (untreated; n = 24) and B (treated; n = 193). Measurements in group B were then subcategorized according to T4 concentrations (reference range 15–50 nmol/L): group B1 = elevated T4 (n = 46), group B2 = normal T4 (n = 84) and group B3 = decreased T4 (n = 63). Group B2 was further divided into cats with low normal (group B2a; n = 35), medium normal (group B2b; n = 29) and high normal (group B2c; n = 20) T4 concentrations.

Results TSH was detectable in 4 (17 %) of the 24 untreated cats (group A) and did not return to normal despite seemingly successful therapy in two. Increased TSH concentrations were observed in 3.6 % of the treated cats in group B2 and 2.9 %, 6.9 %, and 0 % in subgroups B2a, B2b and B2c, respectively. Forty-four percent of the treated cats with a decreased T4 (group B3) had an increased TSH concentration. TSH correlated with treatment length (r = 0.358, p = 0.004) and was significantly higher in cats treated for more than 3 months (p = 0.008).

Conclusion TSH was detectable in a significant number of untreated hyperthyroid cats and thus this parameter should not be used to definitively rule out feline hyperthyroidism. Furthermore, the very low prevalence of increased TSH concentrations in treated hyperthyroid cats with a normal T4 and cost benefit calculations do not support the routine measurement of TSH in these cats. The fact that TSH correlated with time since treatment start and 56 % of the cats with a decreased T4 had TSH concentrations within the reference limits, suggests delayed recovery of the pituitary thyrotrophs which might explain the low prevalence of subclinical hypothyroidism in the present study.

Clinical relevance TSH measurement in cats with suspected or treated hyperthyroidism using a canine assay lacks diagnostic sensitivity and can only complement therapeutic decision-making.

Zusammenfassung

Gegenstand und Ziel In dieser retrospektiven Studie wurde die Eignung eines caninen TSH-Assays zur Diagnose und Therapieüberwachung der felinen Hyperthyreose nach Radiojod- oder Thyreostatikatherapie untersucht.

Material und Methoden Das elektronische Datenerfassungssystem der Veterinärmedizinischen Universität Wien wurde auf Paare von TSH- und T4-Werten bei Katzen durchsucht. 217 Paare von 136 Katzen wurden in die Gruppen A (unbehandelt; n = 24) und B (behandelt; n = 193) unterteilt. Messwerte in Gruppe B wurden anschließend anhand ihrer T4-Konzentrationen subkategorisiert (Referenzbereich: 15–50 nmol/l): Gruppe B1 = erhöhtes T4 (n = 46), Gruppe B2 = normales T4 (n = 84) und Gruppe B3 = erniedrigtes T4 (n = 63). Des Weiteren erfolgte eine Unterteilung der Gruppe B2 in Tiere mit T4 Konzentrationen im unteren (Gruppe B2a; n = 35), mittleren (Gruppe B2b; n = 29) und oberen Drittel (Gruppe B2c; n = 20) des Referenzbereiches.

Ergebnisse Unter den unbehandelten Katzen hatten 4 (17 %) von 24 Tieren ein messbares TSH, welches bei zwei Katzen auch unter scheinbar erfolgreicher Therapie nicht in den Normalbereich zurückkehrte. Bei 3,6 % der therapierten Katzen in Gruppe B2 sowie bei 2,9 %, 6,9 % und 0 % der jeweiligen Subgruppen B2a, B2b und B2c wurde ein erhöhtes TSH beobachtet. Vierundvierzig Prozent der behandelten Katzen mit erniedrigtem T4 zeigten ein erhöhtes TSH. TSH korrelierte mit der Dauer der vorangegangenen Therapie (r = 0,358, p = 0,004) und war signifikant höher bei Katzen mit einer Therapiedauer über 3 Monate (p = 0,008).

Schlussfolgerung TSH ist kein verlässlicher Parameter zum Ausschluss der felinen Hyperthyreose. Die niedrige Prävalenz von erhöhten TSH-Konzentrationen und Kosten-Nutzen-Rechnungen sprechen gegen die routinemäßige Bestimmung von TSH bei behandelten Katzen mit normalem T4. Die positive Korrelation von TSH mit der Therapiedauer und normale TSH-Konzentrationen bei 56 % der Katzen mit einer erniedrigten T4 deuten auf eine verzögerte Erholung der thyreotrophen Zellen und erklären möglicherweise die niedrige Prävalenz von subklinischen Hypothyreosen in dieser Studie.

Klinische Relevanz Die TSH-Messung mittels caninem Assay bei Katzen mit vermuteter oder behandelter Hyperthyreose ist wenig verlässlich und kann die therapeutische Entscheidungsfindung nur ergänzen.

Key words

Cat - subclinical hypothyroidism - endocrinology - internal medicine

Schlüsselwörter

Katze - subklinische Hypothyreose - Endokrinologie - Interne Medizin

Publication History

Received: 09 February 2021

Accepted: 23 June 2021

Article published online:
06 May 2022

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