Maybe reasonable to wait 2 weeks between a contrast CT scan and I131 in cats

Urinary iodine clearance after iodinated contrast administration to healthy cats

Gabriella Allegrini 1, Valerie J Poirier 1, Julia G Pezzali 2, Hughes H Beaufrère 3, Anna K Shoveller 2, Matthew A Kopke 4, Janet Beeler-Marfisi 5

Background
Iodinated contrast media (ICM) is commonly used in computed tomography (CT) scans to enhance imaging. However, exogenous iodine from ICM can interfere with radioactive iodine (¹³¹I) uptake in the thyroid gland, potentially impacting hyperthyroid treatment protocols. This study aimed to determine the time required for urinary iodine concentration (UIC) to return to baseline in healthy cats following ICM administration, providing guidance on the appropriate delay between contrast imaging and ¹³¹I therapy.

Methods
A prospective, single-arm study involved 10 healthy neutered male cats. Cats received Iopamidol (300 mg iodine/mL) at a dose of 2 mL/kg intravenously. Urine samples were collected before and on days 1, 2, 3, 7, 10, and weeks 2–6 after ICM administration. UIC and urinary iodine-to-creatinine ratio (UICR) were measured, and data were analyzed using linear mixed-effects modeling. Cats were kept on a standardized iodine-restricted diet to control for dietary influences on iodine levels.

Results
UIC Response:
-UIC increased 37- to 884-fold on Day 1 after ICM administration and gradually decreased, returning to baseline within 10 days.
-UICR followed a similar pattern, significantly elevated from Days 1 to 10.

Urine Specific Gravity (USG):
-USG remained consistent except in two cats, which experienced transient decreases (<1.035) lasting up to two weeks post-ICM administration, potentially indicating subclinical nephropathy.

Tolerance:
-Iopamidol was well-tolerated, with no observed adverse events.

Limitations
This study was conducted in healthy euthyroid cats; thus, results may not directly translate to hyperthyroid populations. The effect of hyperthyroidism on iodine clearance remains uncertain, as hyperthyroid cats often exhibit increased glomerular filtration rates (GFR), which may accelerate iodine excretion. Serum creatinine was not measured, limiting conclusions about potential subclinical nephropathy.

Conclusions
UIC returns to baseline within 10 days of ICM administration in healthy cats, suggesting that a 2-week delay between iodinated contrast imaging and ¹³¹I therapy is appropriate. Confirmation of this timeline in hyperthyroid cats is needed. The findings support safe clinical protocols for balancing diagnostic imaging and therapeutic interventions in cats.

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