GEP (Life Maintenance) Week 6 Flashcards
Where is the thyroid located vertebrally and what does it consist of
- C5-T1 vertebrae
- Its an endocrine glad
Consists of: - 2 lateral lobes
- Isthmus – 2nd/3rd Tracheal ring
- Pyramidal lobe (occasionally)
-Linked to embryological descent
Name the anatomical structures
Identify these anatomical parts, both arterial and Venous
Arterial supply:
- external carotid artery = supplies superior thyroid artery - supplies superior half of thyroid
- thyrocervical trunk = supplies the inferior thyroid artery – supplies inferior half of thyroid
**inconsistent artery = some people have some don’t = thyroid Ima artery = arises from the brachiocephalic trunk = supplies anterior surface and isthmus
Venous drainage:
- superior and middle thyroid veins = drain into the internal jugular vein
- inferior thyroid vein = drains into brachiocephalic trunk
Innervation:
Branches of the sympathetic trunk
**surgical note of interest = recurrent laryngeal nerve – branch of vagus nerve, runs behind the thyroid in the tracheoesophageal groove
In what area does the thyroid develop during embryology
Area known as the foramen caecum, it descends during development, forming the thyroglossal duct.
At 7 weeks of development the thyroid finishes its descent and reaches its destination in the anterior neck.
Surgical Note: Thyroglossal duct usually regresses at week 10, if still present in adult this can result in thyroglossal cysts
What is the function of the Thyroid
- FUNCTIONS OF THYROID – produces thyroid hormone and calcitonin involved in bone resorption and calcium homeostasis, this is regulated by HPT axis
What is the Hypothalamus-Pituitary-Thyroid-Axis
1) Low blood levels of T3 and T4 OR a low metabolic rate = stimulation of release of TRH (thyrotropin releasing hormone) = TRH is released by the hypothalamus
2) TRH is carried by the portal veins to the anterior pituitary gland
3) Stimulates the release of TSH (thyroid stimulating hormone) by thyrotropes
4) TSH is then released into the blood = stimulates thyroid follicular cells on the thyroid
5) T3 and T4 are then released into the blood by follicular cells
6) Elevated T3 levels inhibit the further release of TRH and TSH = negative feedback
Describe the Synthesis of thyroid hormones
1) Iodide trapping
-TSH binds to TSH receptor on follicular cell
-Stimulated iodide uptake
-Na+/I- symporter
2) Synthesis of thyroglobulin (TGB)
3) Oxidation of Iodide
-Iodide transported into colloid by Pendrin
-Oxidised from Iodide (I-) to Iodine (I0) – by the enzyme Thyroid Peroxidase (TPO)
4) Iodination of Tyrosine
-Thyroid Peroxidase (TPO) catalyses iodination of TGB
-Formation of either:
-Monoiodotyrosine (MIT) = T1
-Diiodotyrosine (DIT) = T2
-Storage in colloid cells
5) Coupling of T1 and T2
-Coupling of T1 + T2 = T3 (More potent)
-Coupling of T2 + T2 =T4 (Majority)
6) Pinocytosis and digestion of colloid
-TSH stimulated
-TGB is cleaved
7) Secretion of Thyroid hormones
-T4 is secreted in greater quantity
-T3 is more potent
-T4 is converted into T3 in peripheries by Iodinases -> transported into the blood
How are Thyroid hormones transported
- Unbound thyroid hormones are the biologically active forms of the hormones -> this is what we measure (free T3 and free T4)
- Thyroid hormones are bound to:
-Thyroid binding globulin (mostly)
-Albumin
-Prealbumin
What are the definitions of Hyper/hypo thyroidism and what does it mean for it to be primary or secondary
- Hyperthyroidism = overactive thyroid (too much T3 / T4)
- Hypothyroidism = underactive thyroid (not enough T3 T4)
- Primary = due to thyroid pathology
- Secondary = due to pathology in the pituitary gland
What is a thyroid function test and the levels you see in the different pathologies of the thyroid
- TFTs can allow us to determine whether someone if hyperthyroid or hypothyroid along with whether the disease is primary or secondary
- Primary hyperthyroidism – TSH is low and T3/T4 is high as the negative feedback loop is working as there is too much T3/T4 so TSH is being inhibited, therefore this is a direct issue with the thyroid gland itself
- Secondary hyperthyroidism - TSH is high which is causing prolonged stimulation ofT3 and T4 causing high levels of both, this shows that the issue lies with the pituitary gland rather than the thyroid as the negative feedback loop is not working
- Primary hypothyroidism - TSH is high and T3/T4 is low, this demonstrates that the thyroid isn’t responding to stimulation from TSH, therefore the issue must be with the thyroid
- Secondary hypothyroidism – TSH is low and T3/T4 is low, the negative feedback loop is constantly in action despite low levels of T3 and T4 showing the problem lies within the pituitary
Different types of hyperthyroidism diseases
TSH secreting pituitary adenoma (0.01%) – benign tumour of the anterior pituitary, overstimulation of the thyroid -> very rare!!
Different types of hypothyroidism diseases
What are other investigations for Thyroids are there apart from Thyroid functiont test
Thyroid Ultrasound
* Useful for people with thyroid nodules
* Can tell you if the nodule is cystic or solid
* Cystic = benign
* Solid = malignant / hyper malignant
* Can be used for fine needle aspiration to look for cancer
-In cases of large solo nodule OR toxic multi nodular goiter
Antibodies in autoimmune disease:
Anti-TSHR = Grave’s disease
Anti-TPO = Hashimoto’s
Anti-TGA = Hashimoto’s
What is the management of hypothyroidism
Only medication to remember -> Levothyroxine. It is a Synthetic T4 which is metabolizes to T3 in the body
Important side note:
* Patients require monthly blood tests to titrate the dose once started
* Once settled blood tests become yearly
Hyperthyroidism Management
- 1st Line – Carbimazole
Prevents the thyroid from producing T3 and T4
Patients then given levothyroxine as a replacement
When given in Grave’s disease, can treat the disease in 4-8 weeks and complete remission and ability to withdraw the drug in 18 months - 2nd Line – Propylthiouracil
Small risk of severe hepatic reactions including death - 3rd Line / Definitive Treatment = Radioactive Iodine Therapy
Single oral dose
The radiation is taken up by the thyroid and destroys several thyroid hormone producing cells:
-can kill too many – cause hypothyroidism which requires levothyroxine to treat*
Not to be used in:
Pregnant women or people planning to get pregnant
Cannot be in close contact with pregnant women or children for 3 weeks
Limit contact with general population for several days
Where is the adrenal gland located
The adrenal gland is located above the kidneys, it is retroperitoneal.
What are the layers of the adrenal gland
dopamine, norepinephrine, and epinephrine are all catecholamines hormones which are produced in the medulla of the adrenal gland
What is the salt aspect of the adrenal gland
This is the zona glomerulosa
What is the sugar aspect of the adrenal gland
This is the zona fasciculata
what is the sex aspect of the adrenal gland
The zona Reticularis
