Thyroid and Parathyroid

Benign and malignant thyroid tumors are common in such patients, and the tumors
. usually are multifocal
Tests of Thyroid Function
( Thyrotropin (TSH, reference range 0.15–4.2 mU/mL
Thyrotropin secretion from the anterior pituitary is controlled via a negative feedback
loop by serum T3 and T4levels. In cases in which high T3 and T4 levels are
encountered (Graves' disease, toxic nodular goiter), TSH levels will be accordingly
low and may be undetectable. When T4levels are low (primary thyroid destruction,
e.g., end-stage Hashimoto's thyroiditis), TSH levels will be correspondingly high.
Many clinicians believe that the circulating level of TSH is the single most sensitive
. test of thyroid function
Older radioimmunoassays have been replaced with more sophisticated immunometric
assays, using monoclonal antibodies that target two separate sites (increasing
specificity) on the TSH molecule. One monoclonal antibody is labeled with a
nonradioactive marker, allowing readings with an accuracy down to 0.005 mU/mL. A
reference range of normal TSH levels has been established in euthyroid patients,
. against which levels from the test subject can be compared
Total Thyroxine (TT4, reference range 55–150 nmol/L) and Free Thyroxine (FT4,
( 12–28 pmol/L
Total thyroxine concentration reflects the fraction of T4bound to TBG and other
carrier proteins in the serum and also the amount of free T4 in circulation. T4
production from the thyroid is dependent on TSH from the pituitary and an adequate
intake of iodine in the diet. When T4production from the thyroid is increased, the
bound and free T4levels rise (FT4 remains in equilibrium with bound T4), resulting in
an increase of TT4. When T4 production from the thyroid decreases, bound and
FT4levels drop, which leads to decreased TT4levels. Conditions leading to a changes
in the level of TBG (e.g., estrogen intake) can alter the level of TT4 (as binding sites
increase), but are not reflected by changes in circulating FT4, and so the individual
. remains euthyroid
Free T4 estimates are not performed as a routine screening tool in thyroid disease.
Use of this test is confined to cases of early hyperthyroidism in which TT4 levels may
be normal but FT4levels are raised. In patients with end-organ resistance to T4
. (Refetoff syndrome) T4levels are increased, but TSH levels usually are normal
Total Triiodothyronine (TT3, reference range 1.5–3.5 nmol/L) and Free
( Triiodothyronine (FT3, 3–9 pmol/L
Levels of total T3 or free T3 are not used as a routine investigation of thyroid
function. FT3 is most useful in confirming the diagnosis of early hyperthyroidism, in
which levels of FT4 and FT3 rise before TT4 and TT3. Most T3 production comes
from the peripheral conversion of T4, and this process may be inhibited by conditions
such as starvation illness (low T3 syndrome) or by the effect of certain drugs (e.g.,
propranolol). There is a rare condition of T3 thyrotoxicosis in which levels of TT4 in
the hyperthyroid patient are normal and radioiodine uptake is normal but the TT3
level is raised. This condition is more common in patients from endemic goiter areas
.
and in patients with small solitary thyroid nodules