A STRAIN DIFFERENCE IN RESPONSIVENESS OF CHICK THYROIDS TO THYROTROPIN AND A STEP-WISE INCREASE DURING THREE YEARS IN THYROID WEIGHTS OF CARNEAU PIGEONS1
White Carneau pigeons aged 6 wks. were obtained from the same commercial hatchery over a period of 5.5 yrs. and thyroid wts. from 862 such birds were obtained 7 to 10 days later. During the first 2 yrs. of observation the avg. thyroid wt. was uniformly close to 40 mg. A step-wise increase in thyroid wt. occurred in the autumns of 1938, 1939 and 1940 without complete return to previous summer levels and early in 1941 an avg. wt. of 179 mg. was obtained. The source or cause of these changes is largely unknown. Incomplete histological data indicate that the later and larger thyroids were functionally more active. 91 detns. of the B. M. R. of individuals of the 2 groups indicate an increase of only 5.5% in the group having larger thyroids. During the last 3 yrs. the thyroids of this widely used strain or type of pigeon have been unsuitable for the bioassay of thyrotrophin though they earlier gave clear evidence of suitability. White Leghorn 2-day chicks from 2 different sources in Indiana, and probably representing 2 hereditary strains, consistently gave quite dis-similar results when used according to the method of Smelser for the bioassay of thyrotrophin. Chicks from the 1 source required 4 times as much thyrotrophin to produce an equal amt. of stimulation, as this is measured by increase of thyroid wt., as did chicks from the other source; and maximum stimulation during a 5-day test yielded thyroids weighing only 15 mg. in the one type of chick and 35 mg. in the other more sensitive type. Since chicks from these 2 sources showed no difference in responsiveness to the gonadotrophin and adrenotrophin contained in various pituitary extracts it is probable that the observed strain or source difference in thyroid responsiveness relates to an actual difference in their thyroids and not to a strain difference in the rate of absorption, excretion or destruction of the injected extract. Irrespective of the animal employed quantitative assays of thyrotrophin require the use of parallel tests with a standard prep. of thyrotrophin.