Describe briefly the physiological consequences of increased thyroid hormone secretion.

Outline:

·        Causes of hyperthyroidism

·        Link effects of hormone to consequences of hyperthyroidism

·        Effects of thyroid hormone on:

- cardiac function

- respiratory function

- metabolic rate and heat production

- metabolism

- growth

Essay:

            The general effect of thyroid hormone is to cause nuclear transcription of a large numbers of genes. Therefore, in virtually all cells of the body, great numbers of protein enzymes, structural proteins, transport proteins and other substances increase. The net result of all this is a generalized increase in functional activity throughout the body. Most of the physiological consequences of increased thyroid hormone secretion can be linked to its actions on the body organs and tissues.

            Hyperthyroidism can be caused by Grave’s disease, ectopic production from neoplasm or ingestion of thyroid hormone. Grave’s disease is an autoimmune disease in which T lymphocytes become sensitized to antigens within thyroid gland and stimulate B lymphocytes to synthesize antibodies to these antigens. These antibodies may activate the thyroid receptors, increase thyroid hormone synthesis and secretion.

            Thyroid hormone increases the metabolic activities of almost all the tissues in the body. The basal metabolic rate (BMR) in hyperthyroidism range from +10 to as high as +100. Due to increased heat production, there is excessive unnecessary sweating and a marked preference to stay indoors (heat intolerance).

            Increased metabolism in the tissues causes more rapid utilization of oxygen than normal and release of vasodilators from the tissue. These effects cause vasodilation in most of the body tissues, thus increasing blood flow. As a result, venous return to the heart increases, which increases cardiac output. In hyperthyroidism, cardiac output sometimes rises to 60 per cent or more above normal. Thyroid hormone increase resting respiratory rate, minute ventilation, and ventilatory responses to hypercapnia and hypoxia. Due to a combination of the above three, increased metabolism, cardiac output and ventilation, the body’s reserve capacity is reduced. Therefore, there is dynspea on exertion.

            Thyroid hormone increases both heart rate and stroke volume. The cardiac inotropic effects are partly indirect, via adrenergic stimulation, and partly direct. Myocardial calcium uptake and adenylyl cyclase activity are increased and enhance contractile force. Thyroid hormone induces the myosin heavy-chain alpha gene, increasing the velocity of myocardial contraction. Hence, in hyperthyroidism, the heart rate is increased (tachycardia). However, concentrations of creatine phosphate are reduced by an excess of thyroid hormone and the heart muscle strength becomes depressed. Therefore, one consequence of severe hyperthyroidism is cardiac decompensation secondary to myocardial failure and increased cardiac load imposed by the increased output. Slight increase in thyroid hormone usually increase contractile strength of muscles, but when the secretion becomes excessive, the muscles become weakened due to excess protein catabolism and hence, muscle weakness is common in hyperthyroid patients.

            Thyroid hormone modulates the activities of the central nervous system in many ways. Thyroid hormone enhances wakefulness, alertness, responsiveness to various stimuli, auditory sense, awareness of hunger, memory and learning capacity. Normal emotional tone also depends on proper hormone availability. Furthermore, the speed and amplitude of peripheral nerve reflexes are increased by thyroid hormone. Therefore, the hyperthyroid individual is likely to have extreme nervousness and many psychoneurotic tendencies, such as anxiety complexes, extreme worry, and paranoia. Because of the exhausting effect of thyroid hormone on the musculature and on the central nervous system, the hyperthyroid subject often has a feeling of constant tiredness, but because of the excitable effects of thyroid hormone on the synapses, it is difficult to sleep. One of the characteristic signs of hyperthyroidism is a fine muscle tremor that is believed to be caused by increased reactivity of the neuronal synapses in the areas of the cord that control muscle tone.

            In addition to increased appetite and food intake, thyroid hormone increases both the secretion of digestive juices and the motility of the gastrointestinal tract.  Therefore, hyperthyroid individuals often experience hyperphagia and diarrhea. In spite of increased intake of food, there is weight loss. This is because thyroid hormone enhances carbohydrate and fat metabolism, increasing the turnover of glucose, lipids, free fatty acids and proteins. Because thyroid hormone increases the quantities of many of the enzymes and because vitamins are essential parts of some enzymes, thyroid hormone causes increased need for vitamins. Therefore, a relative vitamin deficiency can occur when excess thyroid hormone is secreted.

            Thyroid hormone plays an important role in growth by stimulating endochondral ossification, linear growth of bone and maturation of the epiphyseal growth centers. Children with hyperthyroidism is marked by rapid growth with increased bone maturation.

            A common clinical manifestation of hyperthyroidism is exophthalmos (protrusion of eyeballs). This is due to swelling of the extraocular muscles and the connective tissue within the orbit which pushes the eyeballs forward. The exophthalmos is lessened with treatment of hyperthyroidism.