Biological detection of heterozygosity for double muscling in cattle. II. Thyroid hormone concentrations.

Serum levels of triiodothyronine and thyroxine were measured by radioimmunoassay to determine whether differences exist between double-muscled (DM) homozygous, DM heterozygous and non-DM cattle of beef and dairy origins. No differences in triiodothyronine concentrations were found between heterozygous and non-DM beef animals, but thyroxine concentrations in non-DM dairy cattle were significantly different from those in non-DM beef. Unexpectedly, hormone concentrations in DM homozygotes were lower than those in other genotypes.