Iodine is an important component of thyroid hormones, which play active role in several physiological process such as control of basal metabolic rate, oxidation, protein synthesis, immune and muscle functions.
Aetiology: Deficiency of iodine may be primary due to inadequate dietary intake of iodine or may be secondary to conditioning factors such as high intake of calcium or continuous intake of sub-lethal quantities of cyanogenetic glycosides containing plants or plants of Brassica spp including cabbage, rapeseed, turnips and kale. Diets rich in linseed meal and soybean by- products are also considered to be goitrogenic. Pastures heavily dressed with crude-sewage have been incriminated as a cause of secondary goitre in calves. Calcium rich and humus deficient soils are likely to contain relatively low iodine and may predispose iodine deficiency
Clinical findings: Deficiency of iodine causes decreased production of thyroxin and stimulation of thyrotrophic hormones by pituitary gland. This is associated with hyperplasia of thyroid tissue and weakness and hair abnormalities in the affected animals. Signs of loss of libido in males, and failure to express oestrous, abortions and stillbirths in females, and weak new born calves are important signs of iodine deficiency. Affected calves have thick neck and in some cases enlarged thyroid may even obstruct respiration. Alopecia and palpable thyroid gland are found in different species. Alopecia may not be evident in affected foals, but they are too weak to stand and suffer from excessive flexion of the lower forelegs and extension of lower parts of hind limbs. Defective ossification, lameness and deformity of the hock are also evident in foals.
Characteristic signs in pigs include birth of hairless and weak piglets. Most affected piglets die within few hours whereas survivors are lethargic, grow poorly and have abnormal gait due to weakness of legs. Iodine deficiency in adult sheep is marked by thyroid enlargement and increased length of gestation. Newborn lambs are weak and have extensive alopecia and palpable thyroid. Clinical signs in goats are similar to those found in sheep. There is alopecia of varied severity and goitre in kids. Enlarged gland in surviving goitrous animals may extend to greater part of neck causing oedema. Palpation over jugular furrow may reveal presence of a murmur and thrill termed as ‘Thyroid Thrill’.
Diagnosis: Iodine deficiency can be easily diagnosed if goitre is present. Estimation of iodine in blood and milk are reliable laboratory tests. Organic or protein bound iodine in serum or plasma is used as a pointer to circulating thyroxin. Serum thyroxin (T4), and iodine in pasture and bulk tank milk are used to monitor iodine deficiency.
Treatment and Prevention: Supplementation of iodine in diet at the recommended levels (0.8-1.0 mg per kg DM of feed in lactating cows and 0.1-0.3 mg per kg of DM for pregnant cows) is useful in preventing iodine deficiency. Salts of iodine or mineral mixture can be used to enhance iodine level in diet in iodine deficient areas. Application of tincture of iodine (cattle 4 ml, and 2 ml in pigs and sheep) to inside of flank or parenteral administration of iodine or drenching pregnant ewes with 280 mg potassium iodide or 370 mg potassium iodate during 4th or 5th month of pregnancy are also used for effective prevention of iodine deficiency. Iodine therapy needs to be instituted with precaution as excess dose may cause toxicity. The minimum toxic levels for calves and pigs are 50 ppm and 400 ppm, respectively.