Rate of thyroxine secretion by male and laying Japanese quail: identification of the radioactive thyroxine degradation component of the multiphasic 131-I curve.

Counting of radioactivity in Japanese quail in vivo showed a rapid loss of 131-I from the body 12-24 h after the i.v. injection of [131-I]thyroxine (T4), followed by a period of slow decrease in counting rates to 96 h. From comparison of these [131-I]T4 curves with curves for 131-iodide-injected birds and from counts on serum and other tissues in vitro it was concluded that, for Japanese quail, the T4 secretion rate should be calculated using serum samples taken during the first 12 h. Using this time period, the parameters measured were: T4 distribution space, laying hens 45-7 and mature cocks 26-7 and mature cocks 26-3 ml/100 g body weight; fractional degradation rate for T4, hens 5-73 and cocks 3-12/day; serum T4 concentration (Tetrasorb-125 method), hens 1-20 plus or minus 0-07 and cocks 1-34 plus or minus 0-05 (S.E.M.) mug/100 ml (n= 16); T4 secretion rate, hens 3-14 and cocks 1-10 mug/100 g/day.