Iodine balance in relation to iodine intake in ponies.

Iodine is an essential trace element for both, humans and animals because iodine is part of the thyroid hormones, which play an essential role in growth and development. The horse has a high sensitivity for iodine: 3 to 5 g iodine/kg body weight (BW)/d is recommended (1), based on the requirement of other species. The feed should contain 0.1 to 0.2 mg/kg dry matter (2). An overdosage of iodine often occurs because of the uncontrolled use of feed supplements in horse feeding practice. Even just an extra of 35 mg iodine per day (adult horses, dry matter intake: 10 kg) can cause severe health risks, leading to enlarged thyroids and lowered triiodothyronine (T3) and thyroxine (T4) values (3). Therefore a clinical indicator for the estimation of the equine iodine intake would be very useful. In humans and dogs (4), renal iodine excretion is used. The objective of this investigation was to find a similar practical indicator for horses. Thus, the following variables were measured: renal iodine excretion, fecal iodine excretion, protein-bound iodine in serum and serum concentrations of the thyroid hormones T3, T4, free triiodothyronine (FT3) and free thyroxine (FT4).