Thyroid Gland Flashcards
What are the three hormones produced by the thyroid gland?
Thyroxine (T4) Triiodothyronine (T3) and Calcitonin.
What controls the synthesis and release of thyroid hormones?
TSH from the pituitary TRH from the hypothalamus and a negative feedback mechanism.
How do thyroid hormones circulate in the blood?
Free form (<1%) is active while >99% is bound mainly to TBG and albumin.
What is the best screening test for hypothyroidism?
TSH level. Increased TSH & decreased T4/T3 indicate primary hypothyroidism while decreased TSH & decreased T4/T3 suggest secondary hypothyroidism.
What additional tests are used in hypothyroidism diagnosis?
TRH stimulation test decreased free & total T4/T3 increased cholesterol increased CK and positive anti-thyroid antibodies in Hashimoto’s thyroiditis.
What is the best screening test for hyperthyroidism?
TSH level. Decreased TSH & increased T4/T3 indicate primary hyperthyroidism while increased TSH & increased T4/T3 suggest secondary hyperthyroidism.
What additional tests are used in hyperthyroidism diagnosis?
Increased free & total T4/T3 T3 thyrotoxicosis (increased T3 normal T4) subclinical hyperthyroidism (decreased TSH normal T3/T4) decreased cholesterol and positive TSI in Graves’ disease.
What happens to thyroid hormones in sick euthyroid syndrome?
Decreased T3 & increased rT3 normal or slightly low TSH normal or low T4 in severe cases and normal/slightly high TSH during recovery.
What is the main function of parathyroid hormone (PTH)?
Increases serum calcium by stimulating bone resorption increasing intestinal absorption and enhancing renal calcium reabsorption while decreasing phosphorus reabsorption.
What are the key lab findings in primary hyperparathyroidism?
Increased serum calcium increased intact PTH decreased serum phosphorus increased alkaline phosphatase and metabolic acidosis.
How is primary hyperparathyroidism differentiated from hypercalcemia of malignancy?
Primary hyperparathyroidism has calcium <13 mg/dL increased PTH normal PTHrP and metabolic acidosis while malignancy has calcium >13 mg/dL decreased PTH increased PTHrP and metabolic alkalosis.
What are the key lab findings in primary hypoparathyroidism?
Decreased serum calcium increased serum phosphorus decreased PTH and normal alkaline phosphatase.
What differentiates pseudohypoparathyroidism from primary hypoparathyroidism?
Pseudohypoparathyroidism has increased PTH with low calcium due to resistance while primary hypoparathyroidism has low PTH and low calcium.
What are common causes of hypocalcemia?
Primary hypoparathyroidism pseudohypoparathyroidism vitamin D or calcium deficiency chronic renal failure and acute pancreatitis.
What are the lab findings in secondary hyperparathyroidism?
Normal or low calcium low phosphorus (except in renal failure where it’s high) increased PTH and increased alkaline phosphatase.
What are the two forms of calcium in blood?
Ionized calcium (50%) is active and protein-bound calcium (50%) is mainly bound to albumin.
How do you calculate corrected calcium for hypoalbuminemia?
Corrected calcium = Serum calcium + 0.8(4 - serum albumin).
What is the thyroid gland?
One of the largest pure endocrine glands in the body.
What is the weight of the thyroid gland?
16-20 grams.
What factors affect the size of the thyroid gland?
Age (increases with age) and sex (larger in females than males).
Where is the thyroid gland located?
In the lower neck just below the larynx on either side of and anterior to the trachea.
How is the thyroid gland structured?
It has two lobes (right and left) connected by the isthmus which lies anterior to the second and third tracheal rings.
What is the extent of each thyroid lobe?
From the oblique line of the thyroid cartilage down to the 5th or 6th tracheal rings.
What are the two capsules of the thyroid gland?
The true fibrous capsule and the pretracheal fascia.
What is the importance of the pretracheal fascia?
It allows the thyroid gland to move upwards and downwards during swallowing.
What is the arterial blood supply of the thyroid gland?
The superior thyroid artery from the external carotid artery the inferior thyroid artery from the thyrocervical trunk of the subclavian artery and occasionally the thyroid ima artery from the aortic arch.
What is the venous drainage of the thyroid gland?
The superior and middle thyroid veins drain into the internal jugular vein while the inferior thyroid vein drains into the innominate vein.
How is the thyroid gland controlled neurologically?
It is under autonomic nervous system control.
What are the nerves related to the thyroid gland?
The external laryngeal nerve and the recurrent laryngeal nerve.
What is the function of the external laryngeal nerve?
It supplies the cricothyroid muscle which tenses the vocal cords to produce a high-pitched voice.
Where is the external laryngeal nerve located relative to the thyroid gland?
It is 1 cm apart from the superior thyroid vessels at the upper pole of the gland.
What is the function of the recurrent laryngeal nerve?
It supplies all muscles of the larynx except the cricothyroid muscle and controls vocal cord mobility.
Where is the recurrent laryngeal nerve located relative to the thyroid gland?
It is closely related to the inferior thyroid vessels near the lower pole of the gland.
What is the importance of the ligament of Berry in thyroid surgery?
It serves as an important landmark for identifying the recurrent laryngeal nerve.
What are the main functions of the thyroid gland?
It synthesizes T3 and T4 which regulate metabolism and support normal physical and mental growth under the control of TSH from the pituitary and TRH from the hypothalamus.
