BODY TISSUES 317 



steroid molecule. Substitution of an hydroxyl (—OH) group for 

 hydrogen in position 1 1 changes the molecule from a mineralocorticoid 

 to a glycocorticoid. Furthermore, only minor additional changes lead 

 to a variety of sex hormones. 



Thyroid gland. This gland consists of two lobes connected by an 

 isthmus, and in man is attached to the trachea on both sides of the 

 larynx. Before the use of iodized table salt became common practice, 

 enlargement of this gland occurred in iodine-deficient areas, with the 

 production of a large lump on the neck. This malady, called goiter 

 or "big neck," was common in the north-central areas of the United 

 States, which are not replenished by iodine carried inland by sea spray. 

 Normal human thyroids (25 to 30 g. total weight) contain from 12 to 

 25 mg. of iodine, whereas glands of goiterous patients show little or 

 no iodine. The amount found in normal thyroids varies with the age, 

 the season of the year, and the iodine content of the food. Goiter is 

 not peculiar to man but occurs also in other animals and has been 

 produced in fish. 



If the thyroid gland does not develop in early life, children become 

 dwarfed and are known as cretins. The condition itself is known as 

 cretinism. These abnormal dwarfs have low mentality, bowed legs, 

 coarse hair, and thick skin. Hypothyroidism in adults leads to a con- 

 dition known as myxedema. Symj)toms include puffy swelling or 

 edema, general depression of all bodily functions, and usually a de- 

 pressed mental condition. These cases usually respond dramatically to 

 the administration of desiccated thyroid glands or thyroid hormone. 

 Hypothyroid cases exhibit abnormally low metabolic rates. 



An abnormally active thyroid gland causes a disease known as ex- 

 ophthalmic or toxic goiter. Hyperthyroidism produces bidging eyes, 

 nervousness, irritability, and insomnia. The metabolic rate is higher 

 than normal, so patients burn their food reserves more rajiidly, lose 

 weight, and may succumb to cardiac failure unless a portion of the 

 overactive gland is removed. 



The chemistry of thyroid hormones is discussed on page 379. The 

 mechanism by which thyroxine or other active derivatives function 

 in controlling metabolism remains unknown. The action of the 

 hormone has not been related to specific biochemical reactions in 

 metabolism. Erythrocyte synthesis is stimulated by consumption of 

 thyroxine, as is the case of lipide metabolism. Development changes 

 such as metamorphosis of tadpoles are affected by extracts of thyroid 

 tissues. Injections at early stages lead to the development of tiny frogs 

 no larger than flies. 



Parathyroid glands. These glands are so closely associated with the 



