Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism.

BACKGROUND

Iatrogenic hypothyroidism can occur after treatment of hyperthyroidism, and is correlated with a reduced glomerular filtration rate in humans and dogs.

HYPOTHESIS

Cats with iatrogenic hypothyroidism after treatment for hyperthyroidism will have a greater incidence of azotemia than euthyroid cats.

ANIMALS

Eighty client owned cats with hyperthyroidism.

METHODS

Two retrospective studies. (1) Longitudinal study of 12 hyperthyroid cats treated with radioiodine (documented as euthyroid after treatment), to assess changes in plasma thyroid stimulating hormone (TSH) concentration over a 6-month follow-up period, (2) Cross-sectional study of 75 hyperthyroid cats (documented as euthyroid) 6 months after commencement of treatment for hyperthyroidism to identify the relationship between thyroid status and the development of azotemia. Kaplan-Meier survival analysis was performed to identify relationships between thyroid and renal status and survival.

RESULTS

Plasma TSH concentrations were not suppressed in 7 of 8 cats with hypothyroidism 3 months after radioiodine treatment. The proportion of cats with azotemia was significantly (P= .028) greater in the hypothyroid (16 of 28) than the euthyroid group (14 of 47). Twenty-eight of 41 cats (68%) with plasma TT4 concentration below the laboratory reference range had an increased plasma TSH concentration. Hypothyroid cats that developed azotemia within the follow-up period had significantly (P= .018) shorter survival times (median survival time 456 days, range 231-1589 days) than those that remained nonazotemic (median survival time 905 days, range 316-1869 days).

CONCLUSIONS AND CLINICAL IMPORTANCE

Iatrogenic hypothyroidism appears to contribute to the development of azotemia after treatment of hyperthyroidism, and reduced survival time in azotemic cats.