Thyroid disorders

• What shape is the thyroid gland?


  Butterfly-shaped
• What does the thyroid gland control in the body?


  It controls the body's metabolism
• What are the two main thyroid hormones secreted by the thyroid gland?


  Thyroxin (T4) and Tri-iodothyronine (T3)
• What is another hormone secreted by the thyroid gland?

  Thyrocalcitonin
• What is the process of synthesis, storage, and release of thyroid hormones?


  1. Hypothalamus secretes TRH.
  2. TRH stimulates anterior pituitary to secrete TSH.
  3. TSH binds to thyroid cell receptors.
  4. Activation of adenyl cyclase increases cAMP.
  5. Increased size, vasculature, and function of the thyroid gland.
• What is the role of TSH in the thyroid hormone synthesis process?


  TSH stimulates the thyroid gland to increase size, vasculature, and function
• What is iodide trapping?


  Active uptake and concentration of iodide by the thyroid gland
• What facilitates iodide trapping in the thyroid gland?

  TSH and ATP
• What percentage of circulating iodide is trapped by the thyroid gland?


  About 50%
• What can inhibit active trapping of iodide?


  Monovalent chemical groups, anaerobic conditions, and digitalis
• What enzyme converts iodide into active elementary iodine?

  Thyroid peroxidase (TPO)
• What is organification of iodine?


  Iodine combines with thyroglobulin to form mono-iodotyrosine (MIT) and di-iodotyrosine (DIT)
• What results from the coupling of MIT and DIT?


  Formation of Tri-iodothyronine (T3) and Tetra-iodothyronine (T4)
• What happens to MIT and DIT inside the cell?


  They are retained and de-iodinated by thyroid de-iodinase
• What is the consequence of a lack of thyroid de-iodinase enzyme?


  Decreased availability of iodine, leading to hypothyroidism
• What regulates the release of T3 and T4?


  Thyroid-stimulating hormone (TSH)
• How are thyroid hormones absorbed when taken orally?


  Irregularly absorbed
• When is it better to take thyroid hormones orally?


  Before meals on an empty stomach
• What is the major part of circulating T3 and T4 bound to?


  Thyroxin-binding globulin
• What percentage of T4 is bound and free?


  99.9% bound and 0.1% free
• What percentage of T3 is bound and free?


  99.6% bound and 0.4% free
• What effect does aspirin have on T3 and T4?


  Aspirin displaces T3 and T4 from plasma protein binding sites
• What happens when there is a loss of plasma proteins?


  Decreased binding sites for T3 and T4, leading to increased free form
• How do estrogens affect T3 and T4 levels?


  Estrogens increase binding of T3 and T4, leading to decreased free form
• What is the fate of thyroid hormones in the body?

  1. Hepatic conjugation with glucuronic acid.
  2. Excretion in bile into the intestine.
  3. Deconjugation in the intestine and reabsorption into the liver (entero-hepatic circulation).
  4. Oxidative deamination and transamination in the kidney and liver.
  5. T4 is converted to T3 by tissue de-iodinase.
• What is the calorigenic effect of thyroid hormones?


  Increased basal metabolic rate leading to hyperpyrexia
• What are the cardiovascular effects of thyroid hormones?


  Vasodilatation and tachycardia
• What gastrointestinal effects do thyroid hormones have?


  Increased motility leading to diarrhea and malabsorption
• What is Levothyroxin-sodium (L-T4) used for?


  Replacement therapy in hypothyroidism
• What is the initial dose of Levothyroxin-sodium for adults with hypothyroidism?


  50 µg/day for 2 weeks
• What is the dose of Levothyroxin-sodium for children with hypothyroidism adjusted according to?


  Age of the child
• What is the composition of Liotrix?


  T4 + T3 in a ratio of 4:1
• What are the therapeutic uses of thyroid hormones?


  • Replacement therapy in hypothyroidism
  • Treatment of myxoedema in adults
  • Treatment of cretinism in children
  • Management of simple non-toxic goiter
  • Treatment of hypercholesterolemia
  • Gynaecological disorders
• What do thioamides do?


  Decrease organification resulting in decreased synthesis of thyroid hormones
• What is the mechanism of action of potassium perchlorate?


  It competes with iodine for uptake and storage by the thyroid gland
• What is the effect of iodide therapy on TSH?


  It reduces the effects of TSH on the thyroid gland
• What is the therapeutic use of radioactive iodine-131?


  Treatment of moderate and severe hyperthyroidism
• What is the half-life of iodine-131?


  8 days
• What are the side effects of radioactive iodine-131 therapy?


  Pain and congestion at the site of the thyroid and hypothyroidism
• What is the role of beta-blockers in hyperthyroidism treatment?


  They protect the heart from tachycardia and decrease anxiety