The effect of superfluous iodine intake and strumigenic factors on histometric parameters of thyroid gland in layers

The effect of a 74-day increased iodine intake in form of KI on weight and histometric parameters of thyroid gland was studied in four groups of layer s (n = 11) of the laying hybrid Brown Hisex at the beginning of the first l aying cycle. Control group (A) received ad libitum complete feed mix N-1 co ntaining 0.3 mg I/kg. Experimental groups received feed mix N-1 enriched wi th iodine as follows: 3.5 mg I/kg feed in group B, 1.3-10 mg in group C and 7-15 mg in group D. Differentiated iodine supplements had statistically si gnificant effects (P < 0.01) on the height of follicular cells-thyreocytes that decreased with the increasing iodine load of layers (A: thyreocyte hei ght 4.11 mu m, B: 3.96, D: 2.67); the effects on follicle increase were ins ignificant. The thyroid weight was not influenced by higher iodine intake ( Tab. II). The state similar to colloid struma was described in some cases i n experimental groups: follicles fully filled with easily stainable colloid and bordered with flat thyreocytes. This parenchyma structure shows a tend ency to hypothyreosis and colloid struma. The superfluous intake of iodine is accompanied by an increased T-3 plasma concentration (4.48 +/- 1.27 nmol /l at the intake of 10 mg I/kg feed; control group 2.69 +/- 1.45 nmol/l onl y) and by more frequent lymphocytic infiltrations of the thyroid correspond ing to findings at spontaneous autoimmune thyreoitis. The effect of differe ntiated additive intake of iodine on the iodine supply to the layer organis m is documented by its average content in egg yolk in mu g/kg of yolk fresh matter (group A: 2 303, B: 4 762, C: 10 614, D: 29 592). The effect of 30- day intake of strumigenic factors (nitrates and rapeseed meal with glucosin olate content of 18.254 mmol/kg) was studied in three groups of layers with five birds in each. The strumigenic effect of 80 mg nitrates/kg feed water was not compensated by supplementation of 0.5 mg I/kg feed and was statist ically significantly (P < 0.01) reflected in an increase of thyreocyte coun ts by 9.6-25.8% (Tab. IV). The supplement of 126 g rapeseed meal (2.3 mmol of glucosinolates) per 1 kg feed N-1 did not significantly influence the th yroid weight and parenchyma (Tab. VI). The histometric characteristic of th e thyroid corresponded to a euthyreoid state. No strumigenic effects of the glucosinolate content in the supplement of rapeseed meal were observed in layers after 30-day feeding when the parallel iodine intake was 0.3-0.8 mg I per kg of feed N1. No clinical signs of disease were recorded during expe riments. The level of iodine supply to the layer organism was evaluated on the basis of its content in egg yolk. Iodine content in yolk was determined after alkaline digestion by a modified method of Sandell and Kolthoff (Bed nar, 1964). Histological sections of the thyroid were stained with hematoxy lin-eosine, histometric parameters were determined in an image analyzer Luc ia.