Thyroid Gland

Summary

The thyroid gland is one of the largest endocrine glands in the human body. The thyroid gland is a highly vascular, brownish-red gland located in the visceral compartment of the anterior region of the neck, spanning the regions between the C5 and T1 vertebrae and encircling the anterolateral trachea . The left and right lobes of the thyroid gland are connected by a central isthmus. Important processes in metabolism, growth, cardiac function, and calcium homeostasis are regulated by the thyroid hormones (thyroxine and triiodothyronine) and calcitonin.

Overview

• Thyroid gland characteristics: butterfly-shaped, endocrine gland composed of two lobes located inferior to the larynx and anterior to the trachea.
• The isthmus connects the right and the left lobes of the thyroid.
• The thyroid gland develops from the fusion of the median thyroid anlage with the two lateral thyroid anlages, which are derived from the pharyngeal pouches.

Function

• Production of thyroid hormones essential for regulating metabolism and growth
• Parafollicular cells (C cells) produce calcitonin, which influences calcium homeostasis by lowering serum calcium (PTH antagonism).

Regulation Axis—Control

• The hypothalamus produces TRH ➜ TRH binds to membrane receptors in the pituitary glands and stimulates the formation and secretion of thyroidin stimulating hormone (TSH)
• TSH is binds to receptors in the thyroid gland, which then receive additional iodine and synthesize triiodothyronine (T3) and thyroxine (T4).
• Negative Feedback
  • T3 and T4 inhibit the secretion of TRH as well as TSH
  • TSH also inhibits TRH secretion.

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