MEDI7111 Lecture Notes - Lecture 2: Follicular Cell, Thyrotropin-Releasing Hormone, Basal Metabolic Rate
Endocrine 2
Didn’t have enough time to complete this week – Physiology is complete and some sections of path are too.
Anatomy of the Thyroid
Gross
The thyroid gland is formed from the foramen caecum at the base of the tongue and migrates down
the thyroglossal duct where it comes to rest over the junction between the larynx and trachea. It is
comprised of two lateral lobes, an isthmus connecting the two lobes anteriorly and occasionally a
pyramidal lobe which extends superiorly from the isthmus (a remnant of the thyroglossal duct). It
has a fibrous capsule which surrounds and penetrates the gland to form the fibrous septa which
divide it into many small lobules. The parathyroid glands are located on the posterior of the thyroid.
Histology
The thyroid gland consists of many closely packed acini (called follicles), in a background of
capillaries and stroma. Each follicle is lined by a single layer of cuboidal epithelial cells and is filled
with colloid, a protein mixture containing mostly thyroglobulin and stored thyroid hormones. When
the gland is inactive, the cells on the periphery are compressed by the large volume of colloid in the
follicle, however when they are active, the amount of colloid is much less and the cells resume their
cuboidal shape. Scattered between the follicles are parafollicular cells (C cells) which secrete
calcitonin.
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Physiology
HPT Axis
TSH
Thyroid stimulating hormone is released from the anterior pituitary gland in response to thyroid
releasing hormone (TRH) from the hypothalamus. TSH enters the systemic circulation and stimulates
thyroid hormone synthesis and release from the thyroid gland as well as thyroid hypertrophy. The
TSH receptor is present in many tissues (e.g. pituitary, thymus, kidney, testis, brain, adipocytes, and
fibroblasts) which becomes more relevant when discussing the pathology of hyper/hyposecretion of
TSH.
Thyroid Hormones
The thyroid hormones are iodine containing amino acids which control the body’s basal metabolic
rate. In neonates/infants, they assist in the development of the CNS, in children they are responsible
for normal skeletal growth and maturation, and in adults they play crucial roles in multiple organ
functions.
Synthesis
The synthesis of thyroid hormones begins with thyroglobulin; a long protein secreted by the
follicular cells with many tyrosine side chains, which is stored in the colloid. Iodine is pumped into
the colloid via the follicular cells to combine with the tyrosine side chains of the thyroglobulin. Each
tyrosine may have one or two iodine molecules bound to it, forming either MIT or DIT, respectively.
Within the colloid, two adjacent MIT/DIT complexes will undergo a coupling reaction to form T3 or T4
with the number indicating the number of iodine molecules involved. These complexes are then
transported back into the follicular cell where the T3 and T4 are cleaved from the thyroglobulin and
released into the blood.
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