Introduction

Iodine is a constituent of the thyroid hormones thryroxine (T4) and triiodothyronine (T3). Iodine deficiency disorders (IDD) is the term used instead of goiter to denote effects of iodine on growth and development. Most forms of iodine originate from the sea via atmospheric movement and currents carrying iodine and depositing on land. Kelp (Norwegian) has generous levels of iodine. Iodine is not required by plants but is needed by animals.

Metabolism

Absorption

Through feeds and water iodine is absorbed throughout the gastrointestinal tract and is transported by plasma proteins.

• Iodine is also absorbed in the lungs and through the skin
• Iodine is secreted in saliva, other gastrointestinal fluids, and breakdown of iodine from hormones is reabsorbed in the digestive tract
• Intestinal parasitic infestations interfere with iodine absorption

Tissue uptake and distribution

• In plasma, iodine is transported to the thyroid
• If iodine levels are good, 10% or less of absorbed iodine is taken up by thyroid whereas if deficient, more than 80% (Stanbury, 1996)
• Thyroxine is synthesized by iodination of thyroglobulin
• Thyroglobulin is the main storage form in the thyroid

The gland produces and releases thyroxine when stimulated by the pituitary thyroid stimulating hormone (STH). STH acts on tyrosine-rich thyroglobulin to make tyrosine available for iodination. TSH stimulates the thyroid to release thyroxine which is transported to all body cells.

Physiological functions

The only known role of iodine is for synthesis of thyroxine and triidothyronine. Thyroxine contains 65% iodine. Multiple functions as regulator of cell activity and growth.

The thyroid hormones are active in

• Thermoregulation
• Intermediate metabolism
• Reproduction
• Growth and development (e.g., cell differentiation)
• Circulation
• Muscle function
• Oxidation of cells

Hyperthyroidism – weight loss, Hypothyroidism – weight gain

Thyroid hormones also influence mental and physical growth and differentiation of maturation of tissues (Mediated by gene expression, Brody, 1999). Effects other glands, neuromuscular functioning, hair and influences metabolism of nutrients. (e.g., minerals and water).

Deficiency

Iodine deficiency disorder (IDD) including goiter can effect horses and iodine deficiency is suggested by some sources as the most widespread of all mineral deficiencies. Goiter usually appears at birth, other signs include weakness, persistent hypothermia, respiratory distress and high neonatal mortality.

There is an increased susceptibility to infectious diseases and respiratory infections upon deficiency, foals may be stillborn or exhibit extreme weakness at birth (e.g., inability to stand and suckle). Iodine deficient mares have abnormal estrus cycles and stallions potentially less reproductive capacity.