Question

The Thyroid

Discussion Assignment:

1. Thyroid hormones exert their effects on cells in a manner similar to steroid hormones; describe the mechanism of action of thyroid hormones.
2. Why would an imbalance in thyroid hormones have such widespread effects on the body?
3. Why was goiter observed in George’s case?
4. Why would calcium homeostasis be impaired in response to removal of the thyroid gland?
5. What could be used to specifically destroy the thyroid gland and have minimal effects on other cells and tissues in the body?

Case Study:

George is a thirty five year old hardware clerk. During his routine physical he casually mentions to his physician that he seems to be sweating more profusely than normal and most rooms that once were comfortable are now too "hot". At home a room that his wife and children find to be comfortable causes him to sweat profusely. George also reports that he seems to be losing weight even though his appetite has increased. He also complains that he has a shortened attention span and that he always wants to be moving around. Despite the fact that he feels fatigued, George claims to have difficulty sleeping and seems to have more frequent bowel movements, occasionally accompanied by diarrhea. The physician discovers George has lost 15 pounds since his last physical. In checking his chart, the physician finds that George has a negative history for chronic illnesses, does not smoke, and has a low risk for cardiovascular disease. He does however have a positive family history for autoimmune diseases. His father suffers from idiopathic thrombocytopenia, his mother has been diagnosed with rheumatoid arthritis and his oldest sister was recently diagnosed with systemic lupus erythematosus.

Results of George's physical examination were within normal ranges with the exception of the following: he demonstrated tachycardia, loud heart sounds, and apparent cardiac arrhythmias accompanied by slight hypertension. These arrhythmias were confirmed by electrocardiogram to be supraventricular in origin. George's eyeballs appeared large and protruding and his hair was fine and soft. He was also beginning to demonstrate some degree of alopecia. George was also observed to have palmar erythema. palpation of the neck revealed the presence of goiter. Results of blood tests indicated elevated concentrations of thyroid hormones (thyroxin and triiodothyronine), hypercalcemia, and decreased circulating concentrations of lipids. Based on the physical characteristics and the results of the blood tests, George's physician suspected that George was suffering from some form of hyperthyroidism and sent him to an endocrinologist to confirm the initial diagnosis.

Following the initial consultation and examination, the endocrinologist ordered tests to determine whether George was indeed hyperthyroid. The test results indicated an elevation in the concentration of thyroid hormones in the blood and the presence of thyroid-stimulating antibodies. These antibodies specifically stimulate the thyroid gland by binding with the thyroid stimulating hormone receptor located on the plasma membrane of the follicular cells of the thyroid gland. Based on these results, the endocrinologist concluded that George had Grave's disease, a form of hyperthyroidism believed to be autoimmune in nature. George was presented with a number of possible treatment options. These included treatment with chemicals (propylthiouracil and methimazole) that decrease the production of thyroid hormones by the thyroid gland, radioisotopic destruction of the thyroid gland by the use of ¹³¹I, and surgical removal of the thyroid gland. After considering all the options, especially the possible effects of radiation on gamete development, George chose surgery. Following successful surgery, George was prescribed synthetic thyroid hormone to ensure that his body was receiving adequate thyroid hormone and told to return within 2 months for a follow-up evaluation of circulating thyroid hormone concentrations. He was also cautioned to carefully monitor his calcium intake.

Subject Pathophysiology

Answer 1

Ans a) The thyroid hormone is produced by the thyroid glands and they help in regulating the energy and metabolism of the body. The T4 or tetraiodothyronine is the main hormone which is released by the thyroid gland which and the T4 is converted into T3 hormone in the peripheral tissues. The thyroid hormone receptor binds with high affinity to the T3 than T4 and hence is more of active form of thyroid hormone. The thyroid hormone binds to specific receptor known as the thyroid hormone receptor and this receptor is activated by the binding of the thyroid hormone. The active form of the enzyme which is T3 binds to these receptors, there is a conformational change in the receptor and this helps in recruiting co-activator proteins and RNA polymerase which helps in transcription of gene. With this it helps in activation of enzymes which helps in wide range of metabolic functions. This turns on many genes which are present within the pituitary gland, liver cell, kidney cell, heart cell, skeletal muscle etc. and hence helps in metabolic function.

Ans b) The thyroid hormones are responsible for taking care of wide range of metabolic and regulatory function. It helps in efficient working of all the organs and ensure that there is proper metabolism. It also helps in determining the rate of respiration, functioning of heart, kidney, liver etc. when there is thyroid imbalance it causes wide range of impact on the body because these hormones are responsible for controlling many metabolic functions and regulation normal growth and development as well as take care of many regulatory activities in the body. It helps in maintaining homeostasis, maintain basal metabolic rate, regulate heart functioning and rate, body weight, muscle functioning, level of cholesterol etc. The thyroid hormones are released in the bloodstream and binds to the thyroid hormone receptor leading to activation of the genes which helps in various functions. When there is imbalance either it will not sufficiently express genes or overexpress it and hence interfere with the normal functioning.

Ans) The goiter is a condition which is caused due to lack of iron in the body and is basically caused due to the enlargement of the thyroid gland causing neck to swell. The goiter is caused due to overactive thyroid gland due to increase in the concentration of thyroid hormones which are thyroxin and triiodothyronine. Since he has over active thyroid, there is high amount of thyroid hormones and the overstimulation of the thyroid gland leads to its enlargement.

Ans) The removal of thyroid gland can impair calcium homeostasis because the thyroid gland are responsible for production of an important hormone known as calcitonin which plays a pivotal role in calcium homoestasis. The calcitonin hormone helps in regulation the build up of the calcium in bone with help of osteoblasts and helps in reducing the concentration of calcium in the blood. The calcitonin also decreases the blood calcium level by excreting more calcium and hence when the thyroid glands are removed, it will not be able to produce calcitonin and hence it will impair the homeostasis of calcium by increasing the level of calcium in blood.

Ans) The radioactive iodine could be used to specifically target the destruction of the thyroid glands and it would not have impact on other cells as there will be iodine present in the body supplied from outside source.