Write an essay on free T3, the most active
thyroid hormone at periphery?? :_Let us first recapitulate about what we recollect on thyroxine-binding globulin(TBG
protein ) as we read in our physiology class :- ?? Ans:-The small
amounts of T3 and T4 that don't bind with thyroxine-binding
globulin (TBG protein ) are referred to as “free.” This free T4 and Free T3 in particular are
active. Most of the T3 and T4 in body bind to thyroxine-binding
globulin(TBG). This is like SHBG(sex hormone binding globulin) which
combines with androgens and only a small part of androgen circulates in active
form (free testosterone).The serum TBG level test measures the amount of TBG
in blood. ... The illness causing the
low TBG level can cause rise of Free T 3 and cause mild symptoms of
hyperthyroidism. Not only Free T3 binds to globulin but the active thyroid
hormones (THs)—thyroxine (T4) and 3,5,3'-triiodothyronine (T3) also combine with transthyretin (TTR, or
prealbumin), and albumin —circulate in blood
by reversibly binding to carrier proteins. Although only 0.3% or less of T3 and
T4 circulates unbound, it is this free hormone fraction that is metabolically
active at the tissue and cellular level.
How many types of TBG are there( globulin & albumin)?? Ans;-The 3 main proteins that carry the
majority (>95%) of THs are thyroxine-binding globulin (TBG), transthyretin
(TTR, or prealbumin), and albumin. A minor proportion of the Thyroxine is bound on serum lipoproteins. Very rarely,
and in the context of
anti-Thyroid antibodies in autoimmune thyroid disease,
immunoglobulins also may bind TH. Thyroxine
binding to TBG is characterized by low capacity but high avidity; the
converse is true, I e, high capacity but low avidity, for TH binding to TTR and
albumin.
Q. How to test TBG : Which
methodology??
Normal Results:The normal result range will vary slightly
depending on the type of technique the laboratory uses. The two main types of
laboratory techniques used for the serum TBG test are electrophoresis and radioimmunoassay. Typically, the results for both types of
test are measured in milligrams per 100
milliliters, or mg/100 mL.
Electrophoresis
method: of determination of TBG
-
During electrophoresis, a lab technician places part of serum
on specially treated paper or a gel-like substance. An electric current then
runs through it. The proteins move along the paper or gel and form bands that
indicate how much of each protein is in the sample. A lab can analyze these
results. If the lab used electrophoresis to test sample, then normal results will range from 10 to 24 mg/100 mL.
Radioimmunoassay
method of determination of TBG
Radioimmunoassay involves
exposing a sample of blood to an antibody. That antibody will attach to TBG. The antibody has a low-level radioactive
isotope attached to it. The lab can then measure the amount of radiation in the
sample, which indicates the amount of TBG in blood. If the lab used radioimmunoassay to
test sample, then normal results will range from 1.3 to 2.0 mg/100 mL .The
exact standards for normal results may vary depending on lab. When to
suspect thyroid disorders?? These symptoms can include:
- constipation
- diarrhea
- changes in sleep pattern, such as insomnia, daytime
fatigue, or prolonged sleep that isn’t refreshing
- dry skin
- puffy skin
- eye problems, such as dryness, irritation, puffiness,
or bulging
- fatigue
- weakness
- hair loss
- hand tremors
- increased heart rate
- sensitivity to cold
- sensitivity to light
- menstrual irregularity
- weight changes
Can the tests of TBG be fallacious?? Ans:--Interestingly, TBG also binds numerous
T4 and T3 analogues and drugs, such as phenytoin, diclofenac, fenclofenac,
meclofenamate, mefenamate, diflunisal, diazepam, salicylates, and milrinone.
Because some of these drugs also bind to TTR (thyroxine binding receptor) and
may displace TH(thyroxine) from the TTR(receptor)
binding site, it is at least
theoretically possible that patients with either partial or complete TBG
deficiency who are treated with these drugs may show some temporary increase in free TH levels..
Q Which drugs cause falsely high TBG levels?? Ans;-Many different medications and drugs can affect TBG levels. Some of these are medications that
are frequently taken , such as aspirin and birth control pills containing estrogen. Other medications that
can affect serum TBG levels include:
- hormones
- opiates
- opioids
- Depakote
- Depakene
- Dilantin
- phenothiazines
- prednisone
Q. These medications
temporarily before TBG test. What is Thyroxine-binding
globulin (TBG) deficiency?? Ans;- It is a nonharmful condition that is either
acquired or inherited. The only known complications associated with TBG
deficiency are those stemming from the primary disorders that cause the
acquired form of this condition. Complications could also potentially result
from erroneously administered treatment if TBG deficiency is misdiagnosed as
another disorder.
Inherited or acquired
variations in the concentration and/or affinity of these proteins may produce
substantial changes in serum total TH levels measured by commercially available
assays. [1] Notably,
these changes do not result in illness (ie, hypothyroidism or hyperthyroidism), because the concentration of
the free TH does not change.
.
When TBG
Deficincy occurs?? Ans:-TBG deficiency
usually accompanies an underlying illness. A low TBG level itself doesn’t cause symptoms. The illness causing
the low TBG level can cause symptoms, however. if Free T3, FreT4 are high . What happens to Thyroid in TBG Deficiency??
Thyroid-binding
globulin (TBG) deficiency does not cause thyroid disease. The homeostatic
mechanism of equilibrium dynamics between TBG-bound and free TH is as follows:
·
First, any decrease in
TBG levels initially increases the concentration of the free hormone
·
Subsequently, the
tendency to cause hyperthyroidism is counterbalanced by the tendency to shut
off TSH secretion and hence decrease the TH secretory rate from the thyroid
gland
·
Finally, the total TH
concentration in the serum decreases until the concentration of the free
hormone is restored to normal
This equilibrium is
achieved extremely rapidly and on a physicochemical level. If chronic, the
decreased extrathyroidal pool of TH may lead to small, transient declines in
circulating free TH levels, thus resulting in transient TSH stimulation of the
thyroid. The latter mechanism may explain the moderate elevation in serum
thyroglobulin levels observed in up to one third of patients with TBG
deficiency. Because TBG deficiency is not an acute process, a state of
resultant hypothyroidism does not occur. Total T4 and T3 may be low in states
of TBG deficiency, but the free T4, free T3, and TSH levels remain normal.
Patients
may have constitutional symptoms unrelated to thyroxine-binding globulin (TBG)
deficiency (eg, fatigue, weight gain, constipation, drowsiness, somnolence, low
energy, dry skin, edema) that prompt them to seek medical advice. These
symptoms are highly common in the general population and usually lead to
extensive investigations, including TFTs and the ultimate diagnosis of TBG
deficiency.
Most individuals with TBG
deficiency are expected to be asymptomatic. Others present to their health-care
provider because of conflicting findings from a thyroid function screening test
(eg, low total thyroid hormone and normal TSH levels).
Identifying medical and
nutritional states that may be associated with a secondary deficiency of TBG is
very important, because this may indicate important coexisting disease. A
family history of TBG deficiency is suggestive of an inherited state.
In physical examination no specific findings are
associated with inherited deficiency of thyroxine-binding globulin (TBG) upon
physical examination. In secondary deficiency of TBG, any clinical findings are
attributable to the underlying illness.
What is thyroxine-binding globulin (TBG) deficiency & its clinical symp ?? Ans:-Serum
TBG Level
interpretations?? The
most important aspect of dealing with thyroxine-binding globulin (TBG)
deficiency is to recognize and correctly diagnose this condition in order to
avoid unnecessary treatment for a mistaken diagnosis of hypothyroidism.
A firm diagnosis of secondary
TBG deficiency may also be important when it indicates the coexistence of a
previously unrecognized or underestimated serious general medical disease.
Prompt evaluation of the possible causative condition is mandatory.
.
D/D
of diagnosis for TBG
deficiency??
Ans:-Differential diagnosis for TBG deficiency
includes euthyroid
sick syndrome and hypothyroidism. It can help
us to diagnose TSH, free T4, and free T3 levels are normal, but total T4 and total T3 levels are low. Serum thymoglobulin
levels are mildly to moderately elevated in one third of patients.
When TBG is decreased?? Ans:-Thyroxine-binding globulin (TBG) levels
vary, and can be interpreted, as follows:
·
These levels are decreased in patients with secondary TBG
deficiency and incomplete acquired deficiency, but they are undetectable in
cases of complete TBG deficiency (males only)
·
The finding of undetectable TBG in female patients denotes
laboratory error or the very rare occurrence of Homozygosity for TBG gene
mutations and TBG mutations in females with Turner syndrome (XO karyotype) . In patients with qualitative defects, the
TBG concentration may be normal
No comments:
Post a Comment