Increased motility of the gastrointestinal tract . In addition to increasing appetite and food intake, thyroid hormones increase both the rate of secretion of digestive juices and the motility of the gastrointestinal tract. Hyperthyroidism often leads to diarrhea, and a deficiency of thyroid hormones can cause constipation.

Excitatory effects on the central nervous system . Thyroid hormones mainly increase the speed of brain processes, although dissociation reactions are possible. Deficiency of iodine-containing thyroid hormones reduces these functions. With hyperthyroidism, a person may seem overly nervous, detect psychoneurotic tendencies, such as a complex of anger, anxiety or paranoid manifestations.

Effect on muscle function . A slight increase in the concentration of thyroid hormones increases the strength of muscle contraction, with excessive production of hormones, the muscles weaken due to excessive breakdown of proteins. Deficiency of thyroid hormones leads to muscle lethargy, slow relaxation after contraction.

Muscle tremors . One of the characteristic signs of hyperthyroidism is muscle tremors. This is not a large-scale tremor, as in Parkinson’s disease or chills, but tremor with a high frequency (10-15 times per second); it can be observed by putting a sheet of paper on the fingers of outstretched hands, while the sheet vibrates. Presumably, the cause of tremor is an increase in the activity of neuronal synapses in the spinal cord that controls muscle tone. Tremor is important in assessing the degree of influence of thyroid hormones on the nervous system.

Effect on sleep . Due to the activating effect of thyroid hormones on the muscles and central nervous system, a person with hyperthyroidism often experiences constant fatigue, but it is difficult for him to fall asleep due to the exciting effect of thyroid hormones on synaptic transmission. On the contrary, extreme drowsiness is characteristic of hypothyroidism, when sleep can last 12-14 hours / day.

Effect on other endocrine glands . An increase in the production of thyroid hormones stimulates the secretion of most other endocrine glands, but this also increases the need for tissue hormones. For example, an increase in thyroxin production everywhere increases the intensity of glucose metabolism and is therefore the reason for a corresponding increase in pancreatic insulin production. Thyroid hormones increase the level of metabolic processes associated with bone formation and, as a result, increase the need for parathyroid hormone. Thyroid hormones increase the rate of inactivation of glucocorticoids in the liver. In accordance with the negative feedback mechanism, this leads to an increase in ACTH production of the anterior pituitary gland, followed by an increase in the production of glucocorticoids by the adrenal cortex.

The effect of thyroid hormones on sexual function . To maintain normal sexual functions, the production of thyroid hormones should be practically within the normal range. In men, a deficiency of thyroid hormones is apparently the cause of the loss of libido. An extremely high level of hormones, however, can cause impotence.

In women, a deficiency of thyroid hormones is often the cause of menorrhagia or polymenorrhea – rapid and excessively heavy menstruation. Sometimes, in some women, a deficiency of thyroid hormones causes menstrual irregularities and even amenorrhea.  

In women with hypothyroidism , as in men, libido is significantly reduced. The picture in women with hyperthyroidism, oligomenorrhea (very meager menses), and sometimes amenorrhea, is even more confusing. 

There is no data indicating a direct effect of thyroid hormones on the gonads; most likely, it is a combination of a direct effect on metabolic processes in the gonads and an excitatory and inhibitory effect mediated by the feedback mechanism, which occur at the level of the adenohypophysis, the hormones of which control sexual functions.