THYROID FUNCTION TESTS (TFT)

 Thyroxine (T4)
 Triiodotyronine (T3)
 Thyroid stimulating hormone (TSH)
 Free T4
 Free T3

 The Iodine is the most important element in
the biosynthesis of thyroid hormones.
 Thyroglobulin acts as a performed matrix
containing tyrosyl groups to which the
reactive iodine attaches to form the
hydroxyl residues of monoiodotyrosine
(MIT) & diiodotyrosine (DIT).

 The coupling of two DIT molecules forms T4.
 The coupling of one DIT molecules & one MIT
molecule results in the formation of T3 or
reverse T3 (rT3).
 Almost all circulating T4 & T3 hormones are
bound to serum proteins (thyroid hormone-
binding proteins).

Formation of T3
Formation of T4

 Only 0.03 % of T4 & 0.3 % of T3 are not bound
to proteins.
 These fractions, called free T4 (FT4) & free T3
(FT3), are the physiologically active portions
of the thyroid hormones.

 T3 is the most biologically active.
 T3 is three to four times more potent than T4.
 T3 is more active because it is not as tightly
bound to the serum proteins as is T4.
 T3 has a greater affinity to target tissue
receptors.

 Regulation of carbohydrate, lipid & protein
metabolism.
 Central nervous system activity & brain
development
 Cardiovascular stimulation
 Bone & tissue growth & development
 Gastrointestinal regulation

Synthesis & negative feedback regulation of thyroid hormones

 The tests used to investigate thyroid
function can be grouped into:
 Tests that establish whether there is
thyroid dysfunction:
 TSH, total T4 & total T3 measurements.

 Tests to know the cause of thyroid
dysfunction:
 Thyroid auto-antibody
 Serum thyroglobulin measurements.
 Thyroid enzyme activities, biopsy of the
thyroid, ultrasound & isotopic thyroid
scanning.

 TSH:
 The single most sensitive, specific & reliable
test of thyroid status.
 In primary hypothyroidism:
 TSH is increased.
 In primary hyperthyroidism:
 TSH is decrease or undetectable.

 Total T4 and Total T3:
 More than 99% of T4 & T3 circulate in plasma bound to
protein
 Both TT4] & TT3] change if TBG alters, e.g. in pregnancy.
 Free T4 and Free T3:
 Free thyroid hormone concentrations are
independent of changes in the concentration of
thyroid-hormone binding proteins.
 More reliable for diagnosis of thyroid dysfunction.

 Thyroid hormones are analyzed by:
 ELISA
 RIA
 Chemiluminescence
 Fluorescent immunoassay

 Primary hyperthyroidism:
 Plasma TSH:
 Decreased due to feedback inhibition on the
pituitary gland.
 Plasma free & total T4 & T3 concentrations are
increased.

 In a very small percentage of hyperthyroid
patients, plasma total T4 & freeT4 are both
normal, whereas both plasma total T3 &
freeT3 are increased; this condition is known
as T3 hyperthyroidism or T3 thyrotoxicosis.

 Plasma TSH: Increased.
 Plasma free T4 & total T4: Decreased
 Plasma free T3 & total T3 measurements are
of no value here, since normal
concentrations are observed.

 Plasma TSH: Abnormal
 Thyroid hormone levels: Normal
 TSH: Low in sub clinical hyperthyroidism.
 TSH: Elevated in sub clinical hypothyroidism.

 Central (pituitary) hypothyroidism:
 TSH & thyroid hormone levels are low.
 Hyperthyroidism due to a TSH secreting
tumor very rarely.
 Plasma TSH is widely used to screen for
congenital hypothyroidism in the neonate.
 The incidence about one in 4000 live births.

 Text book of Biochemistry – DM Vasudevan
 Text book of Biochemistry – U Satyanarayana
 Text book of Biochemistry – MN Chatterjea

THYROID FUNCTION TESTS (TFT)

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