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1. Thyroid Function Test’s
PREPARED BY: DR.C.SUHAS REDDY 1
The thyroid gland is a butterfly-shaped organ composed of two connecting lobes, it produces the
hormones thyroxine (T4) and triiodothyronine (T3). T4 is produced solely by the thyroid gland
while only 20-25% of T3 is secreted directly by the gland. Approximately 80% of the T3 is formed
by hepatic and renal deiodination of T4.T4 has a longer half-life than T3 – approximately 7 days
versus 1 day.
Thyroid hormones have various biological effects. They stimulate the basal metabolic rate and can
affect protein, carbohydrate and lipid metabolism. They are also essential for normal growth and
development.T3 and T4 circulate in human serum bound to three proteins – thyroxine-binding
globulin (TBG), thyroid-binding prealbumin (TBPA) and albumin. Only 0.02% of these hormones
circulate unbound, free to diffuse into tissues. The “free” fraction is the physiologically active
component.
Patients with a normally functioning thyroid gland are referred to be in a euthyroid state. Thyroid
disorders are 4 times more common in women than in men. Hypothyroidism results from a
deficiency of thyroid hormone production, causing body metabolism to slow down. Thyrotoxicosis
results when excessive amounts of thyroid hormones are circulating, and is normally due to
hyperactivity of the thyroid gland (hyperthyroidism).
Thyroid Function Tests
These tests are specific for thyroid status or function. They are categorized into:
HPT- Hypothalamic Pituitary Thyroid ; TSH- Thyroid Stimulating Hormone ;
TRH- Thyroid releasing Hormone
I] measure conc, of
products secreted by
Thyroid gland
IV] Antibodies
detection to
Thyroid tissue
II] Integrity of
HPT axis
III] Intrinsic
Thyroid gland
function
i) Free T4
ii) Total Serum T4
iii) Serum T3 resign
uptake, and
iv) Total Serum T3
i) TSH
ii) TRH
i) Radioactive
Iodine Uptake
i) Antithyroid
Antibodies

2. Thyroid Function Test’s
PREPARED BY: DR.C.SUHAS REDDY 2
I] To measure the concentration of products secreted by Thyroid gland
i) Free Thyroxine (T4)
Normal range: 0.8-1.5 ng/dL
This test measures the unbound T4 in the serum and is the most accurate reflection of the
thyrometabolic status. A decreased free T4 with an elevated TSH is diagnostic of primary
hypothyroidism. Conversely, an increase in free T4 with a TSH of less than 0.10 milliunit/L is
consistent with nonpituitary hyperthyroidism.
ii) Total Serum Thyroxine (T4)
Normal range: 4-12.5 mcg/dL
Although TSH and free T4 are more preferred, total serum T4 continues to remain the preliminary
screening method to assess thyroid function. This test measures both bound and free T4 and is
influenced by any alteration in the concentration or binding affinity of thyroid-binding protein.An
increased total serum T4 may indicate hyperthyroidism, elevated concentration of thyroid binding
proteins or nonthyroid illness. The differential diagnosis for a patient with this elevation includes
nonthyroid illness, especially when other signs and symptoms of thyroid disease are absent or
inconsistent. Similarly, decreased total serum T4 may indicate hypothyroidism, decreased
concentration of thyroid binding proteins or nonthyroid illness (also known as Euthyroid sick
syndrome). In nonthyroid illness, total serum T4 is decreased (or normal), total serum T3 is
decreased, reverse T3 is increased and TSH is normal.
iii) Serum T3 Resin Uptake
Normal Range: 25% to 34%
This test indirectly estimates the number of binding sites on thyroid binding protein, occupied by
T3. In this test, radiolabeled T3 is added to endogenous hormone. This mixture is added to a resin
that competes with thyroid-binding proteins for free hormone. Radiolabeled T3 binds to any free
endogenous thyroid-binding protein. At the saturation point, any free T3 binds to the resin.
iv) Total Serum Triiodothyronine (T3)
Normal Range: 78-195 ng/dL or 1.2-3.0 nmol/L
The total serum T3 test is primarily used as an indicator of hyperthyroidism. It is usually done to
detect T3 toxicosis, when T3 – but not T4 – is elevated. Total serum T3 concentrations can be low
in euthyroid patients with conditions in which the conversion of T4 to T3 is suppressed.
II] To evaluate the integrity of Hypothalamic Pituitary Thyroid axis
i) Thyroid Stimulating Hormone
(Normal range- 0.25-6.7milliunits/L)

3. Thyroid Function Test’s
PREPARED BY: DR.C.SUHAS REDDY 3
TSH is a glycoprotein with two subunits, alpha & beta. The alpha-subunit is similar to those of
other hormones secreted from anterior pituitary like, FSH, HCG, LH. The beat subunit is unique
& render its specific physiological properties.
Hypo-T- Hypothyroidism ; Hyper-T-Hyperthyroidism : TRH- Thyrotropin releasing hormone ; TSH-
Thyroid releasing hormone
ii) Thyrotropin – Releasing Hormone (TRH)
TRH test is a Stimulation test
Algorithm for investigation of thyroid disease using second-third generation TSH assay as an initial
test in patients without pituitary / neuropsychiatric disease
NO
Y
E
S
THYROID STIMULATING HORMONE
Reference range- 0.30-5.00 milliunits/L
Hypo-T
suspect
Borderline
Hypo-T
No metabolic
T-disease
Borderline
Hyper-T
Hyper-T
suspect
free T4 /
3rd G-TSH
free T4
No further
testing
free T4 –
Thyroid Ab
free T4 –
Thyroid Ab
Hypo-T
Symptoms ?
Evaluate hypo-T
-consider
replacement
Probable
Hyper-T,
Consider T131
uptake
TRH
Test /
follow
Follow-
repeat
TSH in 1yr
<0.10 0.10-0.29 0.30-5.00 5.10-7.00 >7.00
If on T4 Rx,
consider ↓
dose
If on T4 Rx,
consider ↓
dose
If free T4 not ↑,
consider free
T3

4. Thyroid Function Test’s
PREPARED BY: DR.C.SUHAS REDDY 4
TRH a hormone secreted by hypothalamus, regulates TSH secretion from the pituitary. TRH test
measures the ability of injected TRH to stimulate the pituitary to release TRH. This is most reliable
indicator of hyperthyroidism patients, where other tests are equivocal, primarily with the older,
less sensitivity TSH assay.
The test is performed by drawing the base line serum TSH concentration then administrating
approximately 200-400 mcg of TRH Intravenously over 30-60 sec. TSH concentration drawn at
30-60 min. Rise of TSH - 5 microunits/mL over baseline is euthyroid state. There will be blunted
/ absent TSH response for hyperthyroidism patient. However a patient of over 40 years of age will
show a increase of < 5 microunits/mL, in depressed patient & patient excess glucocorticoid.
III] To assess intrinsic Thyroid gland function
i) Radioactive Iodine Uptake Test
This test access the intrinsic function of thyroid gland.
This test used to detect the ability of thyroid gland to trap & concentrate iodine & thereby, produce
thyroid hormone. The use of this test is declined because, the test is not specific & its reference
range must be adjusted to local population.
It is a indirect measure of thyroid gland activity & should not be used as a basic screening test for
thyroid function. In patient with normal thyroid function 12%-20% radioactive iodine will
absorbed by gland after 6 hrs and 5%-25% absorbed after 24 hrs.
This test is useful in distinguishing hyperthyroidism caused by subacute thyroiditis with absent or
reduced iodine uptake,
 High radioactive uptake noted in: Thyrotoxicosis, iodine deficiency, post-thyroiditis.
 Low test results occur in: Acute thyroiditis, hypothyroidism, patents on exogenous
thyroid hormone therapy.
This test is affected with body store of iodine. So, patient should be enquired before the test for
the use of iodine containing products. Test is contraindicated in pregnancy.
IV] To detect antibodies to Thyroid tissue
i) Antithyroid Antibodies
Normal range: varies with antibody.
Antibodies that attack various thyroid tissue components can detect in serum of patients with
autoimmune disorders such as Hashimoto’s thyroiditis & Graves disease. In Graves disease,
hyperthyroidism is caused by antibodies, which attack TSH receptors. In autoimmune thyroiditis,
hypothyroidism may be caused by, antibodies will bind with the TSH receptors, thereby blocking
TSH to show its action.
If significant antibodies present in blood agglutination (clumping) occurs. This test based on
dopamine antagonist metoclopramide. Results reported titers of 1:100 are significant & usually

5. Thyroid Function Test’s
PREPARED BY: DR.C.SUHAS REDDY 5
detected during remission. In hashimoto’s thyroiditis-antibodies > 1:10 to thyroglobulin’s are
present in 60 % – 70 % of adult patients, but typically not detected in during remission. Titer
above 1:1000 are found in both Hashimoto’s thyroiditis & Graves disease.