HORMONES Flashcards
What is the key to thyroid hormone synthesis ? Why ?
- serum IODIDE is trapped in thyroid and incorporated into tyrosine
- iodine transport into follicles is a rate-limiting step
How are thyroid hormones produced ?
- thyrotropin releasing hormone (TRH) is secreted from hypothalamus in response to low T3 and T4
- TRH stimulates production of TSH in the anterior pituitary
- TSH binds to follicular cells of thyroid = secretion of triiodothyronine (T3), thyroxine (T4)*, and reverse T3
What is the major secretory product of the thyroid ?
thyroxine (T4)
How do T3 and T4 circulate in the blood ?
Bound to thyroid binding globulin (TBG)*, transthyretin, and albumin
*TBG has the greatest affinity for T4
How are T3 and T4 regulated ?
- free T4 is deiodinated in the liver = active T3
- T3 regulates rate of cellular oxidation (uptake of glucose) and protein metabolism
- increased T3 and T4 inhibit anterior pituitary response to TRH
- decreased thyroid hormones cause increase in TRH and TSH secretion
List 2 thyroid autoimmune diseases
- Graves’ disease; formation of autoantibodies specific to the TSH receptor = hyperthyroidism
- Hashimoto; cell-mediated autoimmunity causing thyroid dysfunction (goitre, thyroiditis, frequently hypothyroidism)
Endogenous Hypothyroidism
- aka Hashimoto thyroiditis
- autoimmune thyroid disease
Exogenous hypothyroidism
- Graves’ disease
- associated with iodine imbalance and various medications
Secondary hypothyroidism
Due to damage/disease of hypothalamus/ pituitary gland
Endogenous hyperthyroidism
- aka Graves’ disease
- associated with thyroid tumors and disorders of hypothalamus/ pituitary gland
Exogenous hyperthyroidism
- associated with viral and bacterial thyroiditis
Describe Non-thyroidal Illness (NTI)
- low total and free T3; elevated REVERSE T3; normal to low TSH
- seen in starvation, sepsis, surgery, myocardial infarction, diabetes…
- lack of conversion of T4 to T3 at the tissues
- suppresses TRH release, reduced T3 and T4 turnover, reduction of T3 at liver, tissue-specific down-regulation of receptors, transporters, and enzymes
What is the most sensitive and specific test for the investigation and management of primary thyroid dysfunction ?
TSH testing; in comparison with T4, TSH is not markedly affected by NTI or drugs
Which subunit of hCG is specific to the placenta ?
β* = specific to placenta
α = TSH, LH, and FSH
What does hCG do ?
hCG acts on corpus luteum to produce progesterone until placenta can produce enough progesterone to maintain pregnancy
Clinical application of qualitative hCG serum
Used to diagnose pregnancy
Clinical Applications of Quantitative serum/ plasma hCG
- date pregnancy
- detect multiple pregnancies (2X in twins)
- assist in detection of ectopic pregnancies
Clinical Applications of increased intact hCG and free β subunit
- Trophoblastic disease
- Germ cell tumours
- Other cancers (biliary, pancreatic)
What is used to monitor treatment and progression of trophoblastic disease ?
Serial measurement of hCG
What is First Trimester Screening used for ?
- determine risk of having a fetus with serious birth defects
Ie. Neural tube defect or Down’s Syndrome - performed between 10 to 13 weeks’ gestation
- hCG in blood = increased
- pregnancy associated plasma a protein A = increased
- Nuchal Translucency Test = increased nuchal space
What results in the First Trimester Screening would indicate a Trisomy ?
- hCG in blood = increased
- pregnancy associated plasma a protein A = increased
- Nuchal Translucency Test = increased nuchal space
What is the Second Trimester Screen ?
- aka Multiples of the Median/ Quad Screen
- prenatal testing to estimate risk of open spina bifida, anencephaly, trisomy 21 and trisomy 18
- Alpha-fetoprotein (AFP) = increased
- Human chorionic gonadotropin (hCG) = increased
- Unconjugated estriol 3 = increased
- Dimeric Inhibin A = increased
What results in the Second Trimester Screening would indicate fetal abnormality ?
- Alpha-fetoprotein (AFP) = increased
- Human chorionic gonadotropin (hCG) = increased
- Unconjugated estriol 3 = increased
- Dimeric Inhibin A = increased
What is MoM ?
- multiple of the median
- statistical value measures how different patient results is from the median of a similar population
- patients with a greater MoM result than the cut-off value will have additional testing
T or F: The cut-off MoM value does NOT have a 100% positive or negative predictive value
TRUE; The cut-off MoM value does NOT have a 100% positive or negative predictive value
