Thyroid Gland

Related Subjects: |Thyrotoxicosis and Hyperthyroidism |Thyroid Storm - Thyrotoxic crisis |Graves Disease (Thyrotoxicosis) |Amiodarone and Thyroid disease |Thyroid Surgery (Thyroidectomy) |Hypothyroidism |Hashimoto's thyroiditis |DeQuervain's thyroiditis |Subacute Thyroiditis |Thyroid nodule |Congenital Hypothyroidism |Thyroid Function Tests and antibodies |Post partum thyroiditis |Sick Euthyroid Syndrome |Thyroid Exam (OSCE) |Thyroid Gland anatomy and Physiology |Thyroid Cancer

The thyroid trap is an ATP driven Iodide pump found in the basal plasma membrane of the follicular cells which transports iodine against its concentration gradient into the colloid. Thyroglobulin rich in tyrosine residues is in the colloid and is available for iodination. Iodination of thyroglobulin is by thyroid peroxidase.

Anatomy

• Thyroid develops as a median endodermal down growth at the base of the tongue.
• The thyroglossal duct connects the thyroid to the foramen caecum at the base of the tongue and in some, this connection remains. In some, this persists as a cyst anywhere along this line.
• The thyroid responds to TSH released by the anterior pituitary. Neonatal congenital hypothyroidism will lead to poor neurological development called cretinism.
• This is easily prevented as a TSH (which will be high) can be measured from a heel prick test at birth.
• The gland is derived from the 3rd pharyngeal pouch and is composed of follicles. This a collection of Cuboidal epithelial cells around a central lumen. The material in the lumen is called colloid made up of thyroglobulin.
• There are also scattered C cells that produce calcitonin. An underactive gland produces reduced T3 and T4 and this leads to an increase in TSH which can cause growth and hypertrophy of the gland known as a goitre.
• Two lateral lobes separated by the isthmus (lies in front of the 2nd to 4th tracheal rings)with occasionally an additional pyramidal lobe
• It is covered in pretracheal fascia which allows it to move up during swallowing
• Is closely related to the parathyroid glands and the recurrent laryngeal nerve which can be damaged in thyroid surgery

Hypothalamus

• Produces thyrotropin releasing hormone TRH a tripeptide
• Released into the portal system to the anterior pituitary

Pituitary

• TRH activates a G-protein mechanism
• Secondary messenger activation of phospholipase C
• Anterior Pituitary basophilic thyrotropes
• Releases TSH into blood stream

Thyroid - Structure

• Contains follicles containing thyroglobulin which contains the thyroid hormones
• C cells which lie in the interstitium between follicles release calcitonin

Function

• TSH activates a G-protein mechanism and results in raised intracellular levels of cAMP in thyroid follicular cells, as well as increased phospholipase C activity.
• Colloid uptake, proteolysis and T4 release occur within minutes!
• Iodide trapping, peroxidase activity, and glucose metabolism (for NADPH production) rapidly rise
• The follicular cells become larger (and more columnar) and proliferate over days
• The thyroid is able to take up and concentrate Iodine to 30-100 times the plasma value
• Iodide uptake and concentration is blocked by thiocyanate and perchlorate)
• Oxidation of I- I + e by Thyroid peroxidase
• This iodination is blocked by Carbimazole and propylthiouracil

End Organ effects

• Peripherally T4 is deiodinated to T3 the more active form
• T4 as a prohormone , a precursor of T3
• T4 is a highly lipophilic hormone
• T3 binds to a nuclear receptor
• Action is to alter protein synthesis

List of effects

• Increased expression of Na/K ATPase;
• increased sarcolemmal calcium uptake
• increased beta-adrenergic receptor levels in myocardium
• increased hepatic production of sex steroid-binding globulin
• Bone: (cartilage ossification, maturation of epiphyses, chondrocyte maturation - thyroid hormone has a direct effect on bone and indirect effects through growth hormone release and IGF-1 action)
• neuronal: cortical growth, axonal and dendritic growth, myelination; T3 is vital for normal brain development, and fetal TSH secretion starts by about 11-12 weeks. The auditory and visual sensory systems appear particularly dependent on T3
• renal: increased renal plasma flow and GFR, Tm in tubules