DMLT (2nd Year) : Thyroid Function Tests - Some basic concepts (U. P. State Medical Faculty syllabus) in English & Hinglish

1
SOME BASIC CONCEPTS OF BIOCHEMISTRY FOR DMLT SECOND YEAR STUDENTS (U.
P. State Medical Faculty syllabus) in English and Hinglish
LESSON 9
[Thyroid function tests (TFT), Thyroid panel]
IN ENGLISH
THYROID FUNCTION TESTS
Thyroid function tests are designed to distinguish hyperthyroidism and hypothyroidism from the
euthyroid state (normally functioning thyroid gland). The tests usually included are serum levels
of Triiodothyronine (T3) and Tetraiodothyronine (Thyroxine,T4) secreted from Thyroid gland, and
Thyroid stimulating hormone (TSH, thyrotropin) , a glycoprotein, secreted from anterior pituitary.
The thyroid gland is a butterfly-shaped endocrine gland located in the lower front of the neck.
The major thyroid hormone secreted by the thyroid gland is thyroxine, also called T4 because it
contains four iodine atoms. To exert its effects, T4 is converted to triiodothyronine (T3) by the
removal of an iodine atom. This occurs mainly in the liver and in certain tissues where T3 acts,
such as in the brain.
[Thyroid also releases a hormone called calcitonin to help regulate calcium levels in our blood
by decreasing it. Calcitonin isn’t grouped into the “thyroid hormone” name, and hence not in
Thyroid function tests.]
Thyroid hormones are derivatives of the amino acid tyrosine bound covalently to iodine.
T4 and T3 circulate almost entirely bound to specific transport proteins. If the levels of these
transport proteins changes, there can be changes in bound and unbound (free) T4 (FT4) and
unbound T3 (FT3) hormones. Free thyroid hormone measurements indicate the amount of T4
and T3 available to tissue cells. Free thyroxine affects tissue function in the body, but bound

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thyroxine does not. However, the free hormone is less than 1%. A total T4 and T3 blood test
measures both free and bound thyroxine.
Reverse T3
Reverse T3 is a biologically inactive protein that is structurally very similar to T3, but the iodine
atoms are placed in different locations, which makes it inactive. Some reverse T3 is
produced normally in the body, but is then rapidly degraded. In healthy, non-hospitalized people,
measurement of reverse T3 does not help determine whether hypothyroidism exists or not, and
is not clinically useful.
3,3‘,5’- Triiodothyronine (rT3)
TSH is a peptide hormone produced by the anterior pituitary. It consists of two chains: an alpha
chain and a beta chain. It has a molecular mass of approximately 28,000 Da.
Thyroid hormones help the body use energy, stay warm and keep the brain, heart, muscles, and
other organs working as they should.
The amount of T4 produced by the thyroid gland is controlled by thyroid stimulating hormone
(TSH) secreted by the pituitary gland which inturn is regulated by thyrotropin releasing hormone
(TRH) secreted by hypothalamus of the brain. Thus the thyroid gland is part of the
hypothalamic-pituitary-thyroid axis, and control of thyroid hormone secretion is exerted by
classical negative feedback, as depicted in the diagram below. Thyrotropin releasing hormone
(TRH) from the hypothalamus stimulates TSH from the pituitary, which stimulates the release of
thyroid hormones. As blood concentrations of thyroid hormones increase, they inhibit both TSH
and TRH, leading to "shutdown" of thyroid epithelial cells and thereby the release of thyroid
hormones is prevented. Later, when blood levels of thyroid hormone have decreased, the
negative feedback signal fades, and the system wakes up again.
Methods for thyroid hormones estimation -
*chemiluminescence immunoassay (CLIA)
*radioimmunoassay (RIA)
*enzyme-linked immunosorbent assay (ELISA)
*High-performance liquid chromatography (HPLC) and LC-mass-spectrometry (LC-MS)

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In addition, free thyroxine index (FTI) is also done. FTI is derived from a calculation and
provides an indirect measurement of free T4 levels, based on total T4 levels and thyroid uptake
(T-uptake) expressed as the percentage of hormone unbound to the thyroid-binding protein.
However, direct measurement of free thyroxine (FT4) has replaced the FTI test in most clinical
situations.
Reference interval -
T3 : 0.846 - 2.02 ng/ml
T4 : 51.27 - 140.6 mg/l
TSH : 0.27 - 4.20 uIU/ml
Interpretation of thyroid function tests -
Euthyroid - normal thyroid gland function
Hyperthyroid - overactive thyroid function
Hypothyroid - underactive thyroid function
To assess the thyroid function the TSH level in a blood sample serves as an “early warning
system”.
● Normal TSH level - the thyroid is functioning properly.
● High TSH level - primary hypothyroidism (due to destruction of the thyroid gland).
● Low TSH level - hyperthyroidism or occasionally secondary hypothyroidism (due to
decreased synthesis of TSH by pituitary)
● High TSH and low FT4 or FTI (indirect measurement of FT4) indicates primary
hypothyroidism (due to disease in the thyroid gland).
● Low TSH and low FT4 or FTI indicates hypothyroidism due to a problem involving the
pituitary gland.
● Low TSH with an elevated FT4 or FTI indicates hyperthyroidism.
● High T3 levels are often useful to diagnosis hyperthyroidism or to determine the severity
of the hyperthyroidism.
● T3 testing rarely is helpful in the hypothyroid patient, since it is the last test to become
abnormal.
Condition TSH T4 T3 FT4
Normal (euthyroid) normal normal normal normal
Hypothyroidism (primary) high low low/normal low
Hypothyroidism (secondary) low/normal low low/normal low
Hyperthyroidism low high high high
[Primary thyroid disorders originate in the thyroid gland, while secondary thyroid disorders
originate in the pituitary gland]
Thyroid antibody tests - This test is used to measure the levels of antibodies against thyroid
cell proteins formed sometimes in patients with hypothyroidism or hyperthyroidism. Two
common antibodies are thyroid peroxidase antibody and thyroglobulin antibodies which help in
the diagnosis of Hashimoto’s thyroiditis (underactive thyroid). Another antibody is thyrotropin
receptor antibody (TSHR or TRAb) whose blood levels estimation helps to assess response to
treatment of hyperthyroidism in Graves' disease (overactive thyroid), to determine when it is
appropriate to discontinue antithyroid medication, and to assess the risk of passing antibodies to
the fetus during pregnancy.

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Some precautions -
● If the patient is taking medicines for thyroid conditions (thyroxine) to treat the thyroid
disease, it is recommended that the blood sample be taken before taking the dose for
that day.
● Avoid taking biotin (vitamin B7) 2 days before a thyroid function testing as it interferes in
thyroid assays.
● It is better to collect fasting blood as it has been found that fasting causes higher TSH
compared to tests done later in the day with no fasting and thus mild (subclinical)
hypothyroidism, where the TSH levels are only mildly elevated and FT4 in normal, may
be missed.
● Estrogens, such as in birth control pills, or in pregnancy, cause high levels of total T4
and T3 because estrogens increase the level of the binding proteins.
IN HINGLISH
THYROID FUNCTION TESTS
जैसा कि नाम से विदित होता है ये जाँच यह देखने क
े लिए की जाती है कि thyroid gland सुचारु रूप से अपना
कार्य कर रही है या नहीं I इस प्रकार Thyroid function tests hyperthyroidism और hypothyroidism को
euthyroid state (normally functioning thyroid gland) से भेद करने हेतु (distinguish) बनाए गये हैं I इन
tests में आमतौर पर इनक
े serum स्तर शामिल होते हैं - Triiodothyronine (T3) और Tetraiodothyronine
(Thyroxine,T4) secreted from Thyroid gland, तथा Thyroid stimulating hormone (TSH, thyrotropin)
, a glycoprotein, secreted from anterior pituitary.
Thyroid gland एक butterfly-shaped endocrine gland है जो neck क
े सामने क
े निचले भाग में स्थित होती
है I Thyroid gland द्वारा स्रावित प्रमुख thyroid hormone thyroxine है जिसे T4 भी कहते हैं क्योंकि इसमें
चार four iodine atoms होते हैं I अपना प्रभाव डालने क
े लिए T4 से एक iodine atom को हटा दिया जाता है
जिससे यह triiodothyronine (T3) में बदल जाता है I इस परिवर्तन की प्रक्रिया मुख्य रूप से liver में होती है और
क
ु छ tissues में भी, जैसे - brain में, जहाँ T3 अपना कार्य करता है I
[Thyroid also releases a hormone called calcitonin to help regulate calcium levels in our blood
by decreasing it. Calcitonin isn’t grouped into the “thyroid hormone” name, and hence not in
Thyroid function tests.]

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Thyroid hormones amino acid tyrosine क
े derivatives होते हैं जो iodine से covalently bound रहते हैं:
T4 और T3 circulate करते हैं लगभग पूरी तरह specific transport proteins से bound होकर I अगर इन
transport proteins क
े level बदलते हैं तो bound and unbound (free) T4 (FT4) और unbound T3 (FT3)
hormones में भी बदलाव होते हैं I Free thyroid hormone का स्तर tissue cells को उपलब्ध T4 और T3 की
मात्रा को दर्शाता हैं I Free thyroxine शरीर क
े tissue function को प्रभावित करता है लेकिन bound thyroxine
नहीं I हालांकि, free hormone 1% से भी कम होता है I total T4 और T3 blood test में क
ु ल free और bound
hormone मापते हैं I
Reverse T3
Reverse T3 एक biologically inactive protein है जिसका structure T3 से बहुत मिलता है किन्तु iodine
atoms भिन्न locations पर होते हैं और जो इसे inactive बनाते हैं I थोड़ा reverse T3 शरीर में सामान्य रूप से
बनता है किन्तु फिर तेजी से घट जाता है I जो हॉस्पिटल में भर्ती नहीं हैं, ऐसे स्वस्थ लोगों में reverse T3 की जाँच
से hypothyroidism का पता लगाने में कोई मदद नहीं मिलती है और चिकित्सकीय रूप में भी यह उपयोगी नहीं है
I
3,3‘,5’- Triiodothyronine (rT3)
TSH एक peptide hormone है जो anterior pituitary में बनता है I इसमें दो chains - एक alpha chain और
एक beta chain होती है I इसका molecular mass लगभग 28,000 Da होता है I
Thyroid hormones शरीर को energy इस्तेमाल करने, शरीर को गर्म रखने और brain, heart, muscles एवं
दूसरे organs को जैसा उन्हें काम करना चाहिए वैसा काम करने में मदद करते हैं I Thyroid gland द्वारा
produced T4 की मात्रा pituitary gland द्वारा स्रावित (secreted) thyroid stimulating hormone (TSH)
द्वारा नियंत्रित होती है और TSH brain क
े hypothalamus द्वारा स्रावित thyrotropin releasing hormone
(TRH) द्वारा नियंत्रित होता है I इस प्रकार, thyroid gland hypothalamic-pituitary-thyroid axis का एक
हिस्सा है और thyroid hormone क
े स्राव (secretion) का नियंत्रण classical negative feedback द्वारा होता
है, जैसा नीचे दिए चित्र में दर्शाया गया है I Hypothalamus से release हुआ Thyrotropin-releasing hormone
(TRH) pituitary से release हुए TSH hormone को stimulate करता है और ये TSH, thyroid hormones क
े
release को stimulate करता है I जब thyroid hormones का concentration blood में बढ़ता है तो ये TSH
और TRH दोनों को inhibit करता है जिससे thyroid epithelial cells का "shut down" हो जाता है और thyroid
hormones का release रुक जाता है I बाद में जब thyroid hormones का blood में स्तर कम होता है तो
negative feedback का signal कमज़ोर हो जाता है और यह system thyroid hormones को release करने
हेतु पुनः active हो जाता है I

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Methods for thyroid hormones estimation -
*chemiluminescence immunoassay (CLIA)
*radioimmunoassay (RIA)
*enzyme-linked immunosorbent assay (ELISA)
*High-performance liquid chromatography (HPLC) and LC-mass-spectrometry (LC-MS)
इसक
े अलावा, free thyroxine index (FTI) भी test करते हैं I FTI calculation द्वारा निकालते हैं और यह T4
मापने का एक और indirect तरीका है जो total T4 levels और thyroid uptake (T-uptake) पर आधारित है I
T-uptake, thyroid-binding protein से अबाध (unbound) hormone का प्रतिशत होता है I हालांकि, अब
अधिकतर चिकित्सीय परिस्थितियों में FTI क
े स्थान पर FT4 क
े direct measurement का ही प्रयोग होता है I
Reference interval - The values may vary in different laboratories.
T3 : 80-190 ng/dL
T4 : 5 to 12 μg/dL
TSH : 0.5 - 4.5 mU/L
Interpretation of thyroid function tests -
Euthyroid - normal thyroid gland function
Hyperthyroid - overactive thyroid function
Hypothyroid - underactive thyroid function
To assess the thyroid function the TSH level in a blood sample serves as an “early warning
system”:
● Normal TSH level - the thyroid is functioning properly.
● High TSH level - primary hypothyroidism (due to destruction of the thyroid gland).
● Low TSH level - hyperthyroidism or occasionally secondary hypothyroidism (due to
decreased synthesis of TSH by pituitary)
● High TSH and low FT4 or FTI (indirect measurement of FT4) indicates primary
hypothyroidism (due to disease in the thyroid gland).
● Low TSH and low FT4 or FTI indicates hypothyroidism due to a problem involving the
pituitary gland.
● Low TSH with an elevated FT4 or FTI indicates hyperthyroidism.

7
● High T3 levels are often useful to diagnosis hyperthyroidism or to determine the severity
of the hyperthyroidism.
● T3 testing rarely is helpful in the hypothyroid patient, since it is the last test to become
abnormal.
Condition TSH T4 T3 FT4
Normal (euthyroid) normal normal normal normal
Hypothyroidism (primary) high low low/normal low
Hypothyroidism (secondary) low/normal low low/normal low
Hyperthyroidism low high high high
[Primary thyroid disorders originate in the thyroid gland, while secondary thyroid disorders
originate in the pituitary gland]
Thyroid antibody tests - यह test thyroid cell proteins की antibodies, जो कभी - कभी
hypothyroidism या hyperthyroidism क
े मरीज़ों में बन जाती हैं, क
े स्तर को मापने हेतु किया जाता है I दो
सामान्य antibodies हैं - thyroid peroxidase antibody और thyroglobulin antibodies जो Hashimoto’s
thyroiditis ( underactive thyroid) क
े निदान में मदद करती हैं I एक और antibody है - thyrotropin
receptor antibody (TSHR or TRAb) जिसका blood में स्तर का estimation मदद करता है - to assess
response to treatment of hyperthyroidism in Graves' disease (overactive thyroid), to determine
when it is appropriate to discontinue antithyroid medication, and to assess the risk of passing
antibodies to the fetus during pregnancy.
Some precautions -
● यदि मरीज़ thyroid की बीमारी की कोई दवा ले रहा है तो thyroid function tests क
े लिए blood उस
दिन की dose लेने से पूर्व ही ले लेना चाहिए I
● यदि मरीज़ biotin (vitamin B7) ले रहा हो तो blood लेने क
े 2 दिन पूर्व ही biotin लेना बंद कर देना
चाहिए क्योंकि biotin thyroid assay में interfere करता है I
● It is better to collect fasting blood as it has been found that fasting causes higher TSH
compared to tests done later in the day with no fasting and thus mild (subclinical)
hypothyroidism, where the TSH levels are only mildly elevated and FT4 in normal, may
be missed.
● Estrogens, such as in birth control pills, or in pregnancy, cause high levels of total T4
and T3 because estrogens increase the level of the binding proteins.
Some practice questions
Fill in the blanks -
(a)Thyroid function tests में test किये जाने वाले तीन hormones हैं ______, ______ and ____.
(b)The biologically active forms of T3 and T4 are _______ and _____
(c ) TSH is secreted by anterior ________.
(d) The hormone released by hypothalamus and which controls the secretion of TSH is ___.
(e) Thyroid gland is a part of the hypothalamic-________ -thyroid axis.
(f) The two important methods for thyroid hormones estimation are ________, ________.
(g) The two antibodies used for the diagnosis of Hashimoto's thyroiditis are ________ and
_______.

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(h) The mineral required for thyroid hormone synthesis is ________.
(i) ______ blood sample is preferred for TSH estimation.
(j) The vitamin which interferes in thyroid assays is ________.
(k) Euthyroid means ______ thyroid gland function.
(I) The condition in which the thyroid gland makes more thyroid hormones than the body
needs is called _______.
(m) The condition in which the thyroid gland doesn't make enough thyroid hormones to meet
our body's needs is called ________.
(n) T4 and T3 circulate almost entirely bound to specific ________ proteins.
(o) The unbound forms of T3 and T4 are called _______ T3 and ______ T4.
(p) The other name of T4 is ______.
(q) The chemical name of T4 is _____________.
(r) The chemical name of T3 is ______________.
(s) The other name of TSH is ________.
(t) T4 is converted to _____ by the removal of an iodine atom.
(u) High TSH with low T3 /T4 levels in blood indicate __________.
(v) Low TSH with elevated T4/ T3 levels in blood indicate _________.
(w) Iodine deficiency causes iodine deficiency _______.
Ans. (a) T3,T4,TSH, (b) FT3, FT4, (c ) pituitary, (d) Thyrotropin releasing hormone(TRH), (e)
pituitary, (f) ELISA, CLIA, (g) thyroid peroxidase antibody, thyroglobulin antibodies, (h) iodine, (i)
fasting, (j) biotin, (k) normal, (l) hyperthyroidism, (m) hypothyroidism, (n) binding, (o) free, free,
(p) Thyroxine, (q) 3,5,3´5´-tetraiodothyronine, (r ) 3,5,3´- triiodothyronine, (s) Thyrotropin, (t)
T3,(u) hypothyroidism, (v) hyperthyroidism, (w) goiter
REFERENCES
1. Shivraj G et al. Thyroid function tests: a review. Eur Rev Med Pharmacol Sci 13:
341-349, 2009.
2. American thyroid association https://www.thyroid.org › thyroid-f...Thyroid FunctionTests.
3. Huang HL and Aw TC Thyroid function testing - a review. Annals Thyroid Res1(2):17-22,
2014.
Disclaimer : The pictures given in the text have been downloaded from Google images and I
am thankful to the persons who have uploaded these pictures.
Dr. P. K. Nigam, Ph. D. (Retired Biochemist)

DMLT (2nd Year) : Thyroid Function Tests - Some basic concepts (U. P. State Medical Faculty syllabus) in English & Hinglish

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DMLT (2nd Year) : Thyroid Function Tests - Some basic concepts (U. P. State Medical Faculty syllabus) in English & Hinglish