Thyroid Flashcards
Thyroid gland location
located in neck below larynx in front of trachea
right lobe larger than left
gland size is larger in areas of iodine deficiency
arranged around a central lumen
Colloid space
the central lumen that thyroid cells arrange around as follicles
Thyroid hormones are synthesized…
in follicular cells
stored on thyroglobulin in the colloid space
Thyroperoxidase (TPO)
oxidizes iodine coming into the thyroid gland so it can be added onto the tyrosyl moieties to create thyroid hormones
C-cells
synthesize hormone calcitonin
scattered within connective tissue holding together thyroid follicles
calcitonin
a hormone that helps prevents circulating free calcium level from getting too high
thyroxine (T4)
the major thyroid hormone
released bound to carrier proteins
stimulate cholesterol removal from the blood
tri-iodothyronine (T3)
10x more active than T4
mostly produced in peripheral tissues
released bound to carrier proteins
binds nuclear receptor proteins that initiate synthesis of proteins causing an increase in metabolic rate
stimulate cholesterol removal from the blood
generally follows the same pattern as T4
Negative feedback loop of thyroid hormone production
Thyroid hormones —| TRH —| TSH production
TSH function
stimulates iodine trapping and thyroid hormone synthesis and release of T3 and T4
sufficient dietary iodine leads to…
Decreased sensitivity of pituitary gland to TRH, limited secretion of TSH
insufficient dietary iodine leads to…
inadequate production of T4
pituitary increases its secretion of TSH
persistently high TSH levels lead to hypertropy of the thyrod gland
goitre
hypertrophy of the thyroid gland
Congenital hypothyroidism
lack of development of the thyroid gland OR enzyme defects in the pathway of thyroid hormone synthesis
can cause severe mental retardation
cretinism
severe mental retardation as a result of untreated congenital hypothyroidism
Acquired PRIMARY hypothyroidism
disease in the thyroid gland
Possible causes: removal of thyroid gland, autoimmune, excess intake of anti-thyroid drugs, iodine deficiency
SECONDARY hypothyroidism
pathology in the pituitary gland (not enough secrestion of TSH)
TERTIARY hypothyroidism
pathology in the hypothalamus (not secreting enough TRH)
Clinical features of hypothyroidism
low energy level, easily fatigued weight gain intolerance of cold temperature dry skin and hair hoarseness of voice
Thyroid test profile: hypothyroidism
Low free T4 and T3, TSH can be high (primary) or low (secondary)
Primary Hyperthyroidism
Graves disease (autoantibodies against TSH receptors in the thyroid gland)
Toxic multinodular goiter
Thyroiditis (inflammation of thyroid gland)
Secondary hyperthyroidism
pituitary tumour that excretes TSH
elevated TSH and T3/4
Clinical features of hyperthyroidism
nervousness, hyperactivity weight loss heat intolerance sweating rapid heart rate exophthalmous (bulging eyes)
Test profile: hyperthyroidism
low TSH, high T3T4 (1)
high TSH, high T3/T4 (2)
TSH as a clinical test
highly sensitive, analytically robust
most important for thyroid disease
primary screening test for all thyroid disease
log-linear relationship between T4 and TSH (2x T4 = 100x TSH) inverse
TRH test:
rarely used
used to differentiate between secondary and tertiary hypothyroidism
Thyroglobulin
follicular cell thyroid cancers produce high levels of thyroglobulin
Calcitonin
a tumour marker for medullary (c-clel) carcinoma of the thyroid gland
Anti-thyroid antibodies
ordered in the investigation of hypothyroidism
Anti-TSH receptor antibodies
stimulatory (thyroid stimulating immunoglobins TSI or long acting thyroid stimulators LATS) can cause hyperthyroidism
or
inhibitory (thyroid binding inhibitory immunoglobulins TBII) can cause hypothyroidism
parathyroid glands
four small glands located on or near the thyroid gland
secrete parathyroid hormone (PTH) which plays a role in calcium homeostasis
easily damaged
PTH interpreted with a serum calcium level
