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