Hamniett: The Thyroid Gland 



295 



Effect of Thyroid Deficiency on 

 Mentality 



These observations arc in line with 

 clinical reports of the relative physical 

 and psychic development of cretins. 

 It is a generally accepted theory that 

 cretinism is a result of deficient thy- 

 roid activity. This theory is based 

 on the fact that a high incidence of 

 thyroid disturbance, as expressed by 

 thyroid hypertrophy, is usually found 

 in the region where cretinism exists. 

 and that certain post-mortem exami- 

 nations of cretins show either the 

 entire absence of functioning thyroid 

 tissue, or a thyroid gland of very 

 small size and abnormal structure. 

 One of the marked characteristics 

 of the cretins is the low mentality 

 and usually small size of the skull 

 cavity and brain. These individuals 

 also show a retardation of general 

 growth. However, from a standpoint 

 of function, their physiological and 

 reflex activities as mediated by the 

 spinal cord, are apparently present in 

 the normal state of development, while 

 their psychic or cerebral potentialities 

 are decidedly underdeveloped. 



It is hazardous to associate size 

 development with functional develop- 

 ment in a heterogeneous stock. The 

 rats which were used in my experi- 

 ments, however, came from a stock 

 which was highly homogeneous and 

 the controls were litter controls. It 

 is, therefore, not an expansion into 

 improbability to consider that along 

 with the greater retardation of brain 

 growth there was a greater retardation 

 of brain function. If this assumption 

 is correct the differential effects on the 

 central nervous system of thyroid 

 deficiency experimentally produced in 

 albino rats, are quits analogous to the 

 effects of a thyroid deficiency on the 

 same structures in cretins. 



The Thyroid and Sex 



In addition to the difference in re- 

 tardation of brain and spinal cord in 

 one and the same animal there is ex- 

 hibited a sex difference in that in the 



females the retardation of the brain 

 is more marked than in the males. 

 It might ])e noted here that this sex 

 difference in develo])mental rate is 

 not confined to the brain but is a gen- 

 eral i)henomenon exhibited l)\' practi- 

 cally all the organs. 



This sex difference is shown in the 

 greatest degree in the response of the 

 gonads to the thyroid deficiency. The 

 development of the testes and ovaries 

 of the thyroidless rats is given in 

 Figure 2 in terms of that of these 

 organs in their respective controls. 



It is evident that the development 

 of the male gonads is but little re- 

 tarded by the lack of thyroid secre- 

 tion, while the development of the 

 female gonads is not only absolutely 

 inhibited, but retrogressive changes 

 occur and weight is lost. The actual 

 values mean that the growth of the 

 testes in the male thyroidless rats was 

 fifty-one per cent of that of the con- 

 trols for the same age period, while the 

 ovaries of the female thyroidless rats 

 actually lost in weight over twice as 

 much as was gained by their controls. 



This specific sex difference points 

 to the reason why thyroid disturb- 

 ances, particularly goiter, are more 

 frequent in women than in men. 

 Clinical observations show that coin- 

 cident with alterations in the physio- 

 logical activity of the ovaries, such as 

 occur at puberty, menstruation, preg- 

 nancy, and the menopause, there are 

 often changes in the size of the thy- 

 roid. No such cyclic changes in thy- 

 roid size have been demonstrated in 

 men. Correlating these phenomena 

 with those observed in the develop- 

 mental differences in the gonads of the 

 two sexes in the albino rat, it is evi- 

 dent that in the male there is little 

 interrelationship between the thyroid 

 gland and the testes, while in the 

 female a high degree of association 

 is shown to exist. 



This difference in response of the 

 gonads of the two sexes to the thy- 

 roid function suggests that if thyroid 

 defects are transmissible, the line of 



