Thyroid function tests are commonly done to diagnose hyper- and hypo-thyroidism. Older tests such as basal metabolic rate, serum cholesterol, and radioactive iodine uptake are non-specific and have been replaced by more sensitive assays of serum T3, T4, FT3, FT4 and TSH. Measurement of thyroperoxidase antibodies is also useful to diagnose autoimmune causes of hypothyroidism like Hashimoto's thyroiditis. The TRH stimulation test can help distinguish between thyroid disorders of hypothalamic or pituitary origin.

1. Thyroid Function
Tests
R. C. Gupta
M.D. (Biochemistry)
Jaipur (Rajasthan), India

2. Thyroid function tests are commonly done
to diagnose hyper- and hypo-thyroidism
Most patients with thyroid disorders have
only isolated signs and symptoms
A clinical diagnosis is usually difficult
Diagnosis often requires laboratory tests

3. Dietary iodide is absorbed by the alimentary
tract, and released into circulation
Some of it is excreted in urine
Some is taken up by the thyroid gland, and
is converted into iodine
Production of thyroid hormones

4. Iodine reacts with the tyrosine residues of
thyroglobulin
Tyrosine is converted into mono-iodo-
tyrosine (MIT) and di-iodo-tyrosine (DIT)
Two DIT residues react with each other to
form tetra-iodo-thyronine (thyroxine or T4)
One residue each of MIT and DIT react to
form tri-iodo-thyronine (T3)

5. T3 and T4 are stored in thyroid acini,
attached to thyroglobulin
The hormones are released into blood
as and when needed
They are transported in circulation by
thyroxine-binding globulin (TBG)

6. The TBG-bound hormones are inactive
Small amounts of T3 and T4 are released
from TBG
These exist as free T3 and T4 in circulation

7. Free T3 and T4 are the biologically active
forms of the hormones
T4 is more abundant than T3 but T3 is
more powerful

8. Synthesis and release of T3 and T4 are
increased by thyroid-stimulating hormone
(TSH) of anterior pituitary
TSH secretion is increased by thyrotropin-
releasing hormone (TRH) of hypothalamus
High blood levels of T3 and T4 cause feed-
back inhibition of TSH and TRH secretion

10. Before the advent of radio-immuno-
assay for hormones, tests for thyroid
function were indirect and non-specific
Some of the older tests are:
• Basal metabolic rate
• Serum cholesterol
• Serum protein-bound iodine
• Radioactive iodine uptake

11. Thyroid hormones stimulate all metabolic
activities
This increases the basal metabolic rate
(BMR)
Hence, BMR is raised in hyperthyroidism
and lowered in hypothyroidism
Basal metabolic rate

12. However, BMR is affected by a number of
physiological and pathological conditions
Therefore, this test lacks specificity

13. Serum cholesterol level is decreased in
hyperthyroidism and increased in hypo-
thyroidism
But this test is non-specific because serum
cholesterol can become abnormal in a
number of other diseases
Serum cholesterol

14. In this test, the iodine bound to serum
proteins is measured
Most of the protein-bound iodine (PBI) is
present in T3 and T4
Serum protein-bound iodine

15. Serum PBI is increased in hyperthyroidism
and decreased in hypothyroidism
However, accuracy of PBI measurement is
poor

16. This test measures the uptake of circulating
iodide by the thyroid gland
A small dose of a radioactive isotope of
iodine (5 µc of Na131I) is given to the
subject orally in a fasting condition
Radioactive iodine uptake

17. The radioactivity over the thyroid gland is
measured by a Geiger Muller counter after
2, 24 and 48 hours
This is a measure of the radioactive iodine
present in the thyroid gland

18. The normal uptake of the ingested dose is:
▪ 1.5-15% at two hours
▪ 25-50% at 24 hours
▪ 15-40 % at 48 hours
Radio-active iodine uptake is increased in
hyperthyroidism and is decreased in hypo-
thyroidism

19. Sensitive and specific methods of
hormone assays are available now
Hence, the older tests have now become
obsolete

20. Thyroid function tests commonly done
now are:
• Serum T3 and T4
• Serum Free T3 (FT3) and Free T4 (FT4)
• Serum TSH
• Thyroperoxidase antibodies (TPO Ab)
• TRH stimulation test

21. Normal range of total T4 in serum is 5-12
µg/dl
It is increased in hyperthyroidism and is
decreased in hypothyroidism
Serum T4

22. The normal range of total T3 in serum is
0.1-0.2 µg/dl
It is increased in hyperthyroidism and is
decreased in hypothyroidism
An increase in serum T3 with normal T4 is
seen in T3 toxicosis
Serum T3

23. Sometimes, serum total T4 may be normal
despite the presence of a thyroid disorder
In such cases, measurement of serum FT4
can clinch the diagnosis
Normal range of serum FT4 is 0.8-2 ng/dl
It is increased in hyperthyroidism and is
decreased in hypothyroidism
Serum Free T4 (FT4)

24. Like serum FT4, serum FT3 may be very
useful in the diagnosis of subtle disorders
of thyroid gland
Normal range of serum FT3 is 0.3-0.6
ng/dl
Serum Free T3 (FT3)

25. Serum TSH can differentiate between
primary thyroid disorders and those
secondary to pituitary/hypothalamic disease
In primary hyperthyroidism, serum TSH is
decreased due to feedback inhibition
In hyperthyroidism secondary to pituitary or
hypothalamic disease, serum TSH is raised
Serum TSH

26. In primary hypothyroidism, serum TSH is
high due to lack of feedback inhibition
In hypothyroidism secondary to pituitary
or hypothalamic disease, it is decreased
The normal range of serum TSH is 0.5-5
µU/ml

27. Hypothyroidism can sometimes occur due
to Hashimoto’s thyroiditis or post-partum
thyroiditis
In such cases, the disease results from
an autoimmune response against thyro-
peroxidase
Thyroperoxidase antibodies
(TPO Ab)

28. The autoimmune disorder can be
diagnosed by detection of thyro-
peroxidase antibodies in serum
Thyroperoxidase antibodies are present
in serum in Hashimoto’s thyroiditis and
post-partum thyroiditis

29. Anterior pituitary is stimulated by TRH
administration
Serum T3, T4 and TSH are measured
after stimulation
This can distinguish between thyroid
disorders of hypothalamic origin and
pituitary origin
TRH stimulation test

30. TRH stimulation test is rarely required in
hyperthyroidism
It is useful in hypothyroidism with low T3,
low T4 and low TSH
In such cases, a response to TRH
stimulation points to hypothalamic origin
of hypothyroidism

31. Subclinical hypothyroidism usually results
from autoimmune thyroiditis
The patients have vague symptoms,
normal serum T4 and high serum TSH
These patients can progress to frank
hypothyroidism later on
Subclinical hypothyroidism

32. It is recommended that these patients
should be screened for TPO antibodies
If TPO antibodies are present, treatment
is indicated
If TPO antibodies are absent, annual
follow up is recommended