Chemical and morphological studies of thyroid function in lactating cattle.

Autopsy studies of the thyroid gland in lactating cattle were combined with chemical tests of thyroid function, i.e. protein-bound iodine (PBI) in serum, and uptake by the erythrocytes in vitro of I-labelled L-triiodothyronine (EU). The study was made on 255 cows from 5 different slaughterhouses in Sweden. To permit a comparison with the chemical results, the thyroids were classified morphologically as normal, slightly changed, and moderately to greatly changed, respectively. The term goitre was used to denote those glands whose weight exceeded 54 g in SRB cows, 51 g in SLB cows, and 44 g in SKB cows. Examination of glands from cows in the Sundsvall region showed 85 % to be the site of histological changes, which were denoted as moderate to marked in 72 % of them. The corresponding figures for the Östersund region were 68 and 28 %, respectively. In the other sampling regions, the figures ranged from 40—52 % and from 21—28 % respectively. Totally 12 thyroids were found to be goitrous; 5 of them derived from the Sundsvall region, and 3 from the Östersund region. All but 2 of the cows with goitre were chemically euthyroid. Cows from the Sävsjö, Östersund and Halmstad regions had significantly lower EU values than cows from the Uppsala and Sundsvall regions. No corresponding difference was present between the various regions with respect to PBI. Cows with morphologically changed glands had higher PBI values than cows with morphologically normal glands. There was a poor correlation between the nature of the histological change and the results of the chemical analyses.