The Iodine Content of the Thyroid of the Fowl with Reference to Age and Sex

It has been shown by Marine and Williams (1908) and Kendall (1914) that iodine may be regarded as the active constituent in thyroid secretion. Thus it is that the relative activity of the thyroid may be assessed by measurement of its iodine content. It is established that the iodine content of the thyroids varies with the diet (Baumann, 1896), general health (Marine and Lenhart, 1922), and season (Riddle and Fisher, 1925; Seidell and Fenger, 1912−13), and that in addition there are individual differences. Riddle and Fisher record further that in the pigeon the size of the thyroid increases with advancing age but found no evidence of sex dimorphism in thyroid size in that particular form.

The thyroids of healthy, freshly killed fowls and mammals were used. The glands were carefully and completely removed, freed from all adventitious tissues and thoroughly desiccated over concentrated H₂SO₄. When perfectly dry the tissue was accurately weighed. In the case of the fowl the complete thyroid tissue of each individual was used, but in the case of the larger mammals only a portion of recorded weight of the desiccated gland was taken. Kendall’s method for the estimation of iodine was employed. In order to obviate seasonal variation the work was completed during the spring. The fowls had been kept under identical conditions and fed on the same rations—oats, maize, middlings, and bran. They were at liberty on grass. The goats were grazing. The rabbits had been fed on hay, cabbage, oats and flaked maize.

The iodine content of the thyroids of certain animals, as estimated by previous workers on this subject, is shown in Table I.

It is noticed that among these figures there is considerable disparity. Some is doubtless due to errors and differences in methods of investigation. Much, no doubt, is the reflection of the facts that the iodine content is an indication of the relative functional condition of the thyroid and that the activity of the thyroid varies with the season, with the phases of the reproductive cycle, with health, with diet, and so forth. It is seen that in the list the iodine content of the thyroid of the bird is greater than in the case of the mammal.

Twelve White Wyandotte cockerels from the same parental pen, of the same age, and all kept under identical conditions, were used. The onset of sexual maturity was determined by histological examination of the testes. It was found that spermatogenesis was active and complete at 14 weeks.

It is seen that at 14 weeks, that is, at the time of sexual maturity, in this stock, the iodine content becomes higher. The suggestion which emerges from these figures is that the iodine content is higher in the sexually mature bird, the increase being associated with the establishment of active spermatogenesis. In the case of the dog, Marine and Williams (1908) record a low iodine content in individuals under 6 months of age.

According to Riddle (1925) the seasons (autumn and winter) of increase in the size of the thyroid are the seasons of diminution in the size of the gonads. He does not maintain, however, that variations in the size of thyroid are paralleled with variations in iodine content.

Fenger (1912−13), in the case of cattle foetuses, found that the iodine content of the female thyroid was higher than that of the male, and suggested, therefore, that the female thyroid was functionally more active than that of the male foetus of the same age. In order to examine this question of a possible sex dimorphism of this kind in the fowl, 14 adult and related bantams (Old English Game) were examined.

The difference between the average iodine contents of males and females is 0.084 ± 0. 058, i.e. 1.4 times its own probable error, and therefore statistically negligible.

These figures afford no evidence that in the case of these fowls a sex dimorphism in the matter of iodine content of the thyroid exists.

In order to examine anew the suggestion which appeared in Table I, that the iodine content of the bird was higher than that of the mammal, I made use of whatever mammalian material became available, and the result of the examination of these specimens is given in Table IV.

The difference between the average iodine contents of birds and mammals is 0.482 ± 0.045, i.e. 10.7 times its own probable error, and therefore statistically significant.

From these figures it would appear to be the case that the iodine content of the thyroid of the bird is indeed higher than that of the thyroid of the mammal. It did not seem reasonable to ascribe any significant part of this difference to differences in reproductive activity, in food, or in general health, and all were adult individuals.

I wish to express my thanks to Dr F. A. E. Crew, who called my attention to the possible interest of this study, for his help during its course.