chapter 16 part 2Recording-2025-03-15T13:44:19.597Z

Thyroid Hormones Overview

• Thyroid Hormones: Two main types are T4 (Thyroxine) and T3 (Triiodothyronine).

  • Although they have similar functions, they serve as different versions of thyroid hormones.

• Receptor Interaction:

  • T3 and T4 can function intracellularly, differing from most amino-acid based hormones which act extracellularly.

  • Thyroid hormones pass through the cell membrane to activate receptors inside the target cells.

Production of Thyroid Hormones

• Iodine Requirement:

  • Iodine is essential for the synthesis of T3 and T4 hormones; a deficiency results in lack of hormone production.

  • Lack of iodine contributes to the risk of utilizing thyrochromine, which can lead to high cancer rates as it affects various body cells.

• Counteracting Radioactive Iodine:

  • Iodine tablets are administered to saturate the body and prevent uptake of damaging radioactive iodine in emergency situations.

• Thyroglobulin and Iodine:

  • Thyroglobulin, present in thyroid colloid, combines with iodine to synthesize T3 and T4.

  • The combined output of T3 and T4 is secreted into the bloodstream for distribution.

Effects of Iodine Deficiency

• Hormonal Levels:

  • Insufficient iodine leads to low levels of T3 and T4.

  • The reduced hormone levels trigger increased secretion of Thyroid Stimulating Hormone (TSH) and Thyrotropin-Releasing Hormone (TRH), in an attempt to stimulate production.

• Thyroglobulin Buildup:

  • As iodine is still lacking, thyroglobulin accumulates in the thyroid gland, leading to an enlargement known as a goiter.

  • This thyroid enlargement is visibly noticeable in the neck region.

Graves' Disease

• Autoimmune Response:

  • In Graves' disease, antibodies mistakenly stimulate the thyroid gland to produce excess hormones instead of destroying it.

  • Symptoms include exophthalmos (bulging eyes), indicative of increased fluid retention and hormone production.

• Calcium Regulation:

  • The disease also influences calcium metabolism, leading to increased calcium reabsorption and affecting the overall calcium balance in the body.

Calcitonin and Parathyroid Hormone (PTH)

• Calcitonin Function:

  • Released when blood calcium levels are high to help decrease those levels.

  • Secretion of calcitonin is essential for calcium regulation by inhibiting osteoclast activity and promoting bone formation.

• Parathyroid Hormone (PTH) Function:

  • Released when blood calcium levels are low, promoting the increase in blood calcium by stimulating bone resorption and affecting kidney function.

• Regulation Feedback:

  • High Blood Calcium: Calcitonin is secreted.

  • Low Blood Calcium: PTH is secreted.

• Hyperparathyroidism:

  • Excessively high PTH leads to calcium leaching from bones causing weakened, deformed bones and potential neurological effects like flaccid paralysis due to depressed nervous system activity.