Thyroid and Parathyroid

Thyroid and Parathyroid

1. Thyroid

   • Butterfly shaped structure in front of the trachea. Secretes T3 and T4 and calcitonin
   • Components
     • Contains C-Cells, which produces calcitonin Colloid: (water plus protein)Follicular Cells, synthesize thyroglobulin and secretes it into colloid to accumulate iodide.
   • Control
     • Control in the hypothalamus by the Thyrotropin Releasing Hormone (TRH). It releases it to the Thyroid stimulating hormone in the anterior pituitary. It then promotes iodide uptake, promotes synthesis of thyroglobulin and secretes T3 and T4
   • Hormones
     • T3 and T4 hormones. Synthesized from tyrosine. Composition secreted is 90% T4 and 10% T3, T3 is more potent than T4 in target tissues however it degrade faster.
     • Action
       • Unlike the other amines, the T3 and T4 are hydrophobic. Hence it is transported in blood via Tyroxine binding glubulin (TBG) Half life of T3 is 24 hrs while T4 is 8 days. It increases basal metabolic rate like effects of glucagon.  Increased heart rate. Stimulates the Neonatal CNS. Most of it the skin and epithelial linings, for the turnover and replacements.
       • Inhibition
         • Hypothalamus and anterior pituitary has a receptor to detect excess stimulation. It is also inhibited by the dopamine (prolactin) or the grown hormone inhibitory hormone (GHIH). Stimulated by estrogen or extreme cold (Only neonates)
   • Accumulating Iodide
     • Iodide (Comes from diet i.e seafood) ions travel from blood vessels into the follicular cells via the Sodium-iodide co-transporter . In the follicular cell, the iodide concentration is very high and it perfuses thru diffusion into the colloid whereby the Thyroid peroxidase changes iodide to iodine and maintain the "low" concentration of iodide.
     • Synthesis of hormone
       • Thyroglobulin has tyrosine attached, then Iodide residues attach to the tyrosine. It then was endocytosized by the follicular cells and broken down by lysosomes to get T3 or T4.
   • Follicular Cell
     • Action and looks change according stimulus of TSH. With more stimulation, there are more endocytosis and fusion vescicles and lysosomes and Endoplasmic reticulum are all expressed. With below normal stimulation , the apical surface is quite short, with less endocytosis and smaller stuff.
   • Dysfunction
     • Hyperthyroidism. Graves' Disease, autoimmune response to TSH receptor causing  swelling of eyes and upper lip retraction. Enlargement of thyroid glands and localised oedema Hypothyroidism. mostly due to Low dietary iodine. but also there could be autoimmune response (no TSH signalling) Premature ageing is one of the signs
   • 'C' Cells
     • Produces calcitonin (peptide hormone with 32 aa) It has a calcium sensing receptor protein on the "C" cells. If there are high calcium content it stores it as bone. and lowers plasma calcium content.

2. Parathyroid Glands

   • 4 parathyroid glands. Consists of 2 cell types: Chief cells(parathyroid hormone) Oxyphillic cells
   • PTH
     • Parathyroid hormone. When it senses low circ calcium levels, the chief cells in the parathyroid glands secrete PTH and it breaks down of bone and makes kidney function a little differently by absorbing calcium more instead of excreting it out. They work hand in hand with hte calcitonin and parathyroid.