Thyroid function tests (TFTs) are the most frequently ordered endocrine investigations in children and adolescents. Abnormalities in TFTs can help in diagnosis of primary thyroid disorders (i.e. disorders in which the defect is at the thyroid level) as well as secondary or central thyroid disorders (in which defect is at the pituitary level).

1. Thyroid Function Tests
Rajendra Dev Bhatt, PhD Scholar
Asst. Professor/Clinical Biochemist
Clinical Biochemistry & Laboratory Medicine
Fellow: Translational Research (2018-2022) in CVD in Nepal,
NHLBI & NIH, USA

2. Hypothyroidism is the most common thyroid
disorder in the pediatric (as well as adult)
population.
In recent years, there appears to be an increase in
the use of TFTs by clinicians during routine
health assessments.

3. Introduction
Thyroid function tests (TFTs) are the most
frequently ordered endocrine investigations in
children and adolescents.
Abnormalities in TFTs can help in diagnosis of
primary thyroid disorders (i.e. disorders in which
the defect is at the thyroid level) as well as
secondary or central thyroid disorders (in which
defect is at the pituitary level).

4. Thyroid Gland
The thyroid gland, located
immediately below the
larynx on each side of and
anterior to trachea, is one
of the largest of endocrine
glands.
It secretes two major
hormones Thyroxine and
triidothyronine.
It also secretes calcitonin,
an important hormone for
calcium meatabolism

5. Thyroid Hormones
• Thyroid hormone are potent regulator of
cellular proliferation and metabolic rate and
must be maintained within an optimal range
for normal development and health.
• Thyroid hormone metabolism, which describes
the biochemical activation and inactivation of
thyroid hormones, is a powerful mechanism
regulating thyroid hormone action.

6. Thyroid Hormone Synthesis and
Plasma Transport
• Thyroid hormone secretion is regulated by the
hypothalamic-pituitary-thyroid axis.
• Hypothalamus produces thyrotropin releasing
hormone (TRH), which stimulates the pituitary
to secrete thyrotropin. Thyrotropin, also called
thyroid stimulating hormone (TSH), stimulates
thyroid hormone synthesis and glandular
secretion.

8. TRH Release (Hypothalamus)

9. TSH Release (Pituitary)

10. Thyroid Hormone Synthesis
(Thyroid Gland)

11. Thyroglobulin Production
The thyroid follicular cells produce
a protein called thyroglobulin and
release it into the colloid, a process
known as exocytosis.
The colloid is rich in thyroglobulin
as a result.
The TSH that is released from the
anterior pituitary gland will travel
through the bloodstream and bind
to TSH receptors on thyroid
follicular cells.
When TSH binds to follicular cells,
the production of thyroglobulin is
enhanced.

12. • The production of the thyroid hormone is
initiated by the absorption of iodine from the
GI tract, where the iodine is reduced to
iodide and released into the plasma.
• Thyroid follicular cells are uniquely adapted
to concentrate iodine and to incorporate it
into thyroid hormones.

13. Iodide Uptake
• We mainly get iodine from our diet, which then gets absorbed from
the intestines into the bloodstream as iodide.
• In addition to enhancing thyroglobulin production, TSH also
stimulates the follicular cells to increase iodide uptake from the
blood.
• The concentration of iodide is greater in the follicular cells than the
blood, so iodide uses the help of sodium to travel against its
concentration gradient.
• The phenomenon is known as iodide trapping.
• Sodium and iodide enter the follicular cells via a sodium/iodide
symporter.
• TSH will increase the expression and activity of sodium/iodide
symporters in order to facilitate increased iodide uptake.
• Once in the follicular cell, iodide will then enter the colloid via a
transporter called pendrin.
• Iodide is now in the colloid with thyroglobulin.

15. Iodide Oxidation
• The thyroid uses an enzyme
called thyroid peroxidase (TPO)
to oxidize iodide into iodine.
• This process is known as iodide
oxidation because we are losing
electrons to form iodine.
• TSH also increases the activity
of TPO.
• Now we have our 2 ingredients
to make thyroid hormone:
Thyroglobulin and Iodine.

16. Thyroglobulin Iodination
• Thyroglobulin contains tyrosine amino acids residue.
• The thyroid will again use the enzyme TPO to place
iodine onto tyrosine, a process known as organification
or iodination of thyroglobulin.
• If one iodine is placed on tyrosine, then it becomes
monoiodotyrosine (MIT).
• If two iodine are placed on tyrosine, then it becomes
diiodotyrosine (DIT).
• MITs and DIT molecules then get combined to form T3
and T4, which are the main thyroid hormones.

18. Thyroglobulin Proteolysis
• We now have thyroglobulin containing T3 and T4.
• Therefore, we have to cut the T3 and T4 out of the
protein.
• In order to do this, thyroglobulin exits the colloid and
enters back into the follicular cell, a process known as
endocytosis.
• TSH increases thyroglobulin endocytosis.
• Lysosomes in the follicular cell fuse with the
thyroglobulin endosome.
• The lysosomes contain proteolytic enzymes called
proteases that cleave (split) the T3 and T4 from
thyroglobulin.

20. Thyroid Hormone Release
• T3 and T4 will then exit the follicular cell and enter the
bloodstream to act on target tissues and organs.
• The majority of T3 and T4 are bound to carrier proteins
as they travel through the blood.
• One of the main carrier proteins is thyroxine-binding
globulin (TBG)
• Although T3 and T4 can both have effects on target
tissues, T3 is considered the active form of thyroid
hormone.
• Much of T4 gets converted into T3 peripherally, with
some of it getting converted in the thyroid.

22. • Both active thyroid hormones, thyroxine (T4)
and tri-iodothyronine (T3), exert negative
feedback upon the hypothalamus and the
pituitary.
• Assays to measure serum TSH, T4, and T3 are
all routinely available in clinical laboratories
and are adequate to diagnosis primary and
central thyroid dysfunction.

23. Thyroid hormones are
composed mostly of
iodine (65% of T4’s
weight; 58% of T3’s
weight) which is
primarily derived from
the diet.
In non-pregnant adults, a
daily dietary iodine
intake of 100–150 μg is
sufficient to meet the
synthetic requirements of
the normal thyroid
hormones.

25. • The sodium/iodide symporter (NIS) is a
membrane protein on the basolateral surface of
thyroid follicular cells that actively transports
circulating iodide into the thyrocyte.
• Intracellular iodide is then oxidized and
conjugated onto tyrosine residues of the large
glycoprotein thyroglobulin via a process called
“organification”.

26. • This tyrosine iodination reaction can add either
one iodine atom (resulting in
monoiodotyrosine or MIT) or two (resulting in
diiodotyrosine or DIT), and the subsequent
“coupling” of two iodinated tyrosines produces
both T4 (formed by the coupling of two DIT
molecules) and T3 (formed by the coupling of
one DIT molecule and one MIT molecule).

27. • Mature thyroglobulin thus contains both T4 and
T3, present in a ratio of about 15:1
• Thyroglobulin is stored within the center of
thyroid follicles in a matrix called colloid and
this constitutes a large reservoir of preformed
hormone, equivalent to several weeks of
normal secretion.
• This storage property is unique to the endocrine
glands and permits continued thyroid hormone
availability during transient iodine deficiency
and other environmental goitrogen exposures
which might otherwise temporarily impair
thyroid hormone synthesis.

28. Thyroid hormone secretion begins with the
endocytosis of colloid from the apical membrane
of thyroid follicular cells. Thyroglobulin then
undergoes lysosomal proteolysis to release T4
and T3, which subsequently enter the circulation
at a ratio of about 11 :1

29. Many of the biochemical reactions required for
thyroid hormone synthesis are catalyzed by the
thyroperoxidase enzyme.
In addition, several other thyroid-specific
proteins such as NIS, the thyroid oxidases which
generate H2O2 for iodine oxidation, and the
iodotyrosine deiodinases responsible for iodine
recycling are also required for normal thyroid
hormone synthesis.

30. • Thyroid hormones have poor aqueous
solubility and most T4 and T3 in the
circulation are bound to plasma proteins.
• The major binding protein in humans is
thyroxine-binding globulin, which binds about
68% of circulating T4 and 80% of circulating
T3, followed by albumin and transthyretin
(previously termed thyroxine binding
prealbumin).

31. • Only about 0.02% of serum T4 and 0.3% of
serum T3 is normally free (unbound) and
available to enter cells and signal thyroid
hormone action.
• An understanding of this “free hormone concept”
is important in clinical practice, as several
inherited and acquired conditions can alter the
amount of these serum binding proteins and/or
their affinity for thyroid hormones.

32. • While free thyroid hormone concentrations are
normal in such individuals, these binding
abnormalities can mimic the laboratory findings of
central thyroid disease and care must be taken to
avoid misdiagnosis.
• For example, congenital thyroxine-binding globulin
deficiency presents with a low serum total T4 and
normal serum TSH (mimicking central
hypothyroidism).

33. • Serum thyroid hormone binding proteins serve
two important physiologic functions.
• The first is to prolong the serum half-life of
thyroid hormones by reducing their renal
clearance.
• The second is to promote a homogenous level
of T4 and T3 in the circulation.

36. Regulation of Thyroid Hormone
Synthesis

37. Catabolism of Thyroid Hormones
• T4 has a half-life of 4-7 days, while T3 has
about 1 day.
• T3 is biologically more active. T4 is a
prohormone which is deiodinated to T3. In the
peripheral tissues, de-iodination takes place.
• This is done by a deiodinase, a selenium
containing enzyme.

38. Thyroid Hormone Action
• The hormone attaches to specific nuclear
receptors.
• Then the receptor-hormone complex binds to
the DNA. The T3 receptor complex binding
sequence in the DNA or the thyroid responsive
element (TRE) has been identified
• The T3 binding results in increase in
transcription rate.

39. • Thyroid hormones enter cells through
membrane transporter proteins.
• Once inside the nucleus, the hormone
binds its receptor, and the hormone-
receptor complex interacts with specific
sequences of DNA in the promoters of
responsive genes. The effect of the
hormone-receptor complex binding to
DNA is to modulate gene expression,
either by stimulating or inhibiting
transcription of specific genes.

40. For the purpose of illustration, consider one
mechanism by which thyroid hormones increase the
strength of contraction of the heart.
Cardiac contractility depends, in part, on the relative
ratio of different types of myosin proteins in cardiac
muscle.
Transcription of some myosin genes is stimulated
by thyroid hormones, while transcription of others
in inhibited. The net effect is to alter the ratio
toward increased contractility.

41. Metabolic Effects of Thyroid
Hormones
• Thyroid hormones have profound effects on many
physiologic processes, such as development,
growth and metabolism.
• They stimulate diverse metabolic activities most
tissues, leading to an increase in basal metabolic
rate.
• One consequence of this activity is to increase
body heat production, which seems to result, at
least in part, from increased oxygen consumption
and rates of ATP hydrolysis.

42. Lipid Metabolism:
Increased thyroid hormone levels stimulate fat
mobilization, leading to increased concentrations of
fatty acids in plasma.
They also enhance oxidation of fatty acids in many
tissues. Finally, plasma concentrations of cholesterol
and triglycerides are inversely correlated with
thyroid hormone levels.
One diagnostic indication of hypothyroidism is
increased blood cholesterol concentration.

43. Carbohydrate Metabolism:
Thyroid hormones stimulate almost all aspects of
carbohydrate metabolism, including
enhancement of insulin-dependent entry of
glucose into cells and increased gluconeogenesis
and glycogenolysis to generate free glucose.

44. Protein Metabolism:
Earliest effect of T4 is stimulation of RNA
synthesis and consequent increase in protein
synthesis. Higher concentration of T3 causes
protein catabolism and negative nitrogen
balance.

45. Other Metabolic Effects
On muscle:
T3 increases glucose uptake by muscle cells it
also stimulate protein synthesis and therefore
growth of muscle through its stimulatory actions
on gene expression.
Thyroid hormone sensitizes the muscle cell to
the glycogenolytic actions of epinephrine.
Glycolysis in muscle is increased by this action
of T3.

46. On the pancreas:
Thyroid hormone increases the sensitivity of the
cells of the pancreas to those stimuli that normally
promote insulin release and is required for optimal
insulin secretion.
On Cardiovascular system:
Thyroid hormones increases heart rate, cardiac
contractility and cardiac output. They also promote
vasodilation, which leads to enhanced blood flow to
many organs.

47. On Central nervous system:
Both decreased and increased concentrations of
thyroid hormones lead to alterations in mental state.
Too little thyroid hormone, and the individual tends
to feel mentally sluggish, while too much induces
anxiety and nervousness.
On Reproductive system:
Normal reproductive behavior and physiology is
dependent on having essentially normal levels of
thyroid hormone. Hypothyroidism in particular is
commonly associated with infertility.

48. Hypothyroidism
• Primary hypothyroidism: (Most common): --
‐ Failure of thyroid gland.
• Secondary hypothyroidism: Failure of the
pituitary to secrete TSH (rare).
• Failure of the hypothalamic-pituitary‐thyroid
axis.

49. Causes:
• Hashimoto’s disease.
• Radioiodine or surgical treatment of
hyperthyroidism.
• Drug effects.
• TSH deficiency.
• Congenital defects.
• Severe iodine deficiency.

50. Hyperthyroidism
• Increased secretion of thyroid hormones.
• Tissues are exposed to high levels of thyroid
hormones (thyrotoxicosis).
• Increased pituitary stimulation of the thyroid
gland (secondary).

51. Causes:
• Grave’s disease.
• Toxic multinodular goiter
• Thyroid adenoma.
• Thyroiditis.
• High Intake of iodine / iodine drugs
• Intake of exogenous T4 and T3.

57. TEST YOURSELF
1. The thyroid gland mostly secretes:
A. T3
B. T4
C. Equal quantities
D. Neither
B. T4

58. 2. T4 is mostly converted to T3 in
A. Thyroid
B. When Needed
C. Peripheral Tissues
D. Never converted
C. Peripheral Tissues

59. 3. A patient is presented with diarrhea,
palpitation and weight loss, he most probably
has:
A. Hyperthyroidism
B. Hypothyroidism
C. Euthyroid
A. Hyperthyroidism

60. THANK YOU