Hyperthyroidism: Causes, Symptoms, And Lab Investigations

Hyperthyroidism: Causes, Symptoms, And Lab Investigations

Describe synthesis and action of thyroid hormone add a note on hyperthyroidism.
Answer:

Thyroid hormone Synthesis:

• Stages involved in the synthesis of thyroid hormones are

1. Thyroglobulin synthesis:

• Thyroid follicular cells synthesize thyroglobulin and secrete it into the lumen by exocytosis.
• Synthesis of thyroglobulin occurs continuously.
• After synthesis, the thyroglobulin is stored in the follicle.

2. Trapping of iodide:

• Iodide is transported actively from the blood into the follicular cell against the electrochemical gradient.
• Iodide is carried along with sodium into the follicular cell.

3. Oxidation of iodide:

• Oxidation of iodide occurs inside the follicular cells in the presence of iodide peroxidase enzyme.
• Iodide is oxidized to iodine.

4. Iodination of tyrosine:

• It is accelerated by iodinase enzymes.
• It occurs in several stages.
• Tryosine is iodized to monoidotyrosine (MIT)
• Next, when a second iodine is added it forms diiodotyrosine (DIT).

5. Coupling reactions:

Coupling reactions occurs to give rise to different thyroid hormone.

These reactions are as follows.

DIT + DIT→ T₄ called thyroxine

DIT+ MIT→T₃ called tri-iodo-thyroxine.

Actions of thyroid hormone:

1. On basal metabolic rate (BMR):

• Thyroid hormones increases the metabolic activities of almost all tissues of the body.
• They stimulate heat production during these reactions.
• Thus, they increases BMR by also increasing oxygen consumption by tissues.

2. On metabolism:

1. On protein metabolism:
   • T4 increases protein synthesis.
   • Increases the translation of RNA.
   • It increases the synthesis of RNA.
   • T4 causes protein catabolism due to increase in BMR.
2. On carbohydrate metabolism:
   • T4 increases peripheral utilization of glucose Increases absorption of glucose from GIT.
   • Decreases the rate of secretion of insulin.
   • Increases breakdown of glycogen into glucose accelerates gluconegenesis.
3. On fat metabolism:
   • Increases synthesis of cholesterol in liver.
   • Decreases fat storage.
   • Increases free fatty acid level in blood.
   • Increases breakdown of cholesterol in liver.
4. On vitamin metabolism:
   • If thyroxine secretion is increased, there is decrease in vitamin levels.
   • T4 is required for hepatic conversion of B-corotene to vitamin A and for the conversion of vitamin A to retinine.

3. On body temperature:

• Thyroid hormone induces thermogenesis.

4. On growth:

• T4 is important for normal body growth and skeletal maturation.
• T4 helps in tissue differentiation and maturation.
• T4 promote growth and development of the brain during fetal life and the first few years of postnatal life.

5. On body weight:

• Increase in thyroxine secretion decreases the body weight due to decrease in fat storage.

6. On blood:

• Thyroxine is one of the maturation factor of erythropoiesis.

7. On cardiovascular system:

• T4
  1. Increases heart rate
  2. Increases force of myocardial contraction
  3. Increases cardiac output, thus increases systolic BP.
  4. Produces vasodilatation, thus decreases diastolic BP.
  5. Increases pulse pressure.

8. On respiration:

• Increases the rate and force of respiration.

9. On GIT:

• T4 increases apatite and thereby food intake.
• Increases secretions and movements of GIT.

10. On CNS:

• T4 is stimulating factor for the central nervous system.
• It increases the blood flow to brain.

11. On skeletal muscle:

• Thyroxine induce excess neuronal activity.
• Lack of thyroxine makes the muscles more sluggish.

12. On sleep:

• Hyposecretion of thyroxine causes somnolence.

13. On sexual function:

• Thyroxine is essential for normal sexual function.
  1. Hyposecretion leads to
     • Loss of libido.
     • Menorrhagia and polymenorrhea.
  2. Hypersecretion leads to.
     • Impotence
     • Oligomenorrhea and amenorrhea.

Hyperthyroidism:

It is the condition resulting from increased circulating levels of free T₄ and T₃.

Causes:

• Grave’s disease – autoimmune disease
• Thyroid adenoma – localized tumour
• Presence of TSH like substances.

Features:

• Increased sweating
• Increased motility, catabolism
• Decreased body weight.
• Goitre – enlargement of thyroid gland
• Tachycardia and atrial fibrillation
• Cardiac failure.

LAB Investigation:

• Serum TSH level-normal or decreases.
• Low cholesterol level.
• Polycythemia.
• Hyperglycemia.