Scientific developments since about 1980 have resulted in the availability
of numerous clinical tests for the diagnosis of thyroid dysfunction.
To evaluate a patient efficiently for possible thyroid disease,
the clinician can now select the most appropriate procedure from
a broad test menu1. With societal interest in performance enhancement & the ability to purchase materials worldwide via the internet, facticial hyperthyroidism
(thyrotoxicocis factitia)...factitial derangement must be kept in mind.
[a test decision tree HERE]
Due to the allegedly fairly low frequency of undiagnosed thyroid
disease in the adult population, the costs of various measurements, and
the reluctance of third-party carriers and government programs
to reimburse for multiple test panels (coupled with complexities
in interpretation), it is the prevailing view by many that tests
of thyroid function should not be part of multiphasic screening
for patients who are not expected (they are asymptomatic) to have
thyroid disease.1 We do not fully share that view because
we believe that many patients are unintentionally non-forthright
about symptoms. Therefore, the factitious element needs to be kept in mind.
Tumors and masses info HERE.
High Risk Groups
There are certain high-risk populations:
- the newborn
[for whom screening for congenital hypothyroidism is mandatory];
- individuals with a strong family history of thyroid disease;
patients (especially hyperthyroidism in those age 60 or more);
- postpartum women four to eight weeks after delivery;
- and patients with autoimmune disease1.
- Also (among the "walking well", persons who are unintentionally losing weight and those
with dry skin and lack of energy constitute two groups with increased
prevalence of thyroid disease).
Screening tests are improving
Screening and diagnosis of thyroid disorders have improved considerably
with the development of increasingly sensitive assays. Each succeeding
generation of assays, especially as applied to thyroid-stimulating
hormone (TSH) testing, has demonstrated a 10-fold increase in functional
sensitivity (lowest concentration an assay can attain with an interassay
CV of <20 percent).2 Third-generation TSH
assays now have a functional sensitivity of 0.01 IU/mL, which enables
differentiation of the hyperthyroid population. These tests emerged
in the late 1980s due to significant advances in nonisotopic (non-RIA) immunoassays.
To provide appropriate standardization, recombinant TSH (rTSH)
has been prepared; and studies have been completed to validate
the material as the first World Health Organization reference reagent
for TSH3. Rule of thumb as to sensitivity: a two-fold change in free T4 will cause a 100-fold change in TSH19.
Thyroid hormones (T3 and T4) exert a broad
range of effects on development, growth and metabolism. The clinical
manifestations of thyroid hormone excess and deficiency are dramatic
examples of the myriad actions of the hormone. Thyroxine (T4),
the primary secretory product of the thyroid, is relatively inactive
and is converted to the active hormone, triiodothyronine (T3),
by the enzyme thyroxine 5'-deiodinase. The actions of "thyroid
hormone" are primarily the result of interaction of T3 with
nuclear receptors for T3 that bind to regulatory genes
and modify their expression.4 T3 also increases
growth hormone synthesis in laboratory animals. In adults with
hypothyroidism (underactive thyroid), stature is reduced in proportion
to the duration of hypothyroidism in childhood. Their basal serum
growth hormone concentrations are normal, but their responses to
provocative stimuli, such as thyroid-releasing hormone, are impaired.
Nocturnal secretion of growth hormone is decreased, as are serum
insulin-like growth factor I (IGF-I) concentrations. In hypothyroid
adults, the serum concentrations and bioactivity of IGF-I appear
to be reduced.4 Triiodothyronine (T3) stimulates
the production of IGF-I through direct effect on the liver and
the stimulation of growth hormone.4
Hormones of all types are divided into two classes determined
on the basis of their physical-chemical characteristics (aqueous
and hydrophobic). The protein and peptide hormones are soluble
in aqueous solutions, unlike the hydrophobic (small
molecule) target hormones, which exist in solution by virtue of
being bound by albumin and specific transport proteins.
The protein and peptide hormones, such as growth hormone, the
pituitary gonadotrophins, TSH, ACTH, melanocyte-stimulating hormone
(MSH), insulin and glucagon all turn over rapidly with half-times
of about 20 minutes or less. In contrast, the hydrophobic small
molecules have longer half-lives, ranging in normal men from about
an hour for cortisol to about a week for thyroxine. Ordinarily,
serum thyroxine remains constant for long periods in health and
disease. Changes are almost always gradual, occurring over weeks
or months.5 Thyroid hormone stimulation of protein synthesis
occurs at the DNA transcriptional level, which takes longer to
have metabolic impact.
More than 30 years ago thyrotropin-releasing hormone (TRH) was
isolated and characterized from bovine and porcine hypothalamic
tissue as a tripeptide (pyroglutamyl-histidyl-proline amide), which
could stimulate the release of thyrotropin (TSH) from the mammalian anterior
pituitary.6 It can also stimulate prolactin secretion
from the normal pituitary gland.6
TRH stimulates TSH release after attachment to high-affinity
pituitary receptors, activation of adenyl cyclase and subsequent
generation of cyclic AMP. The secretion of TSH is primarily regulated
by the negative-feedback suppression of thyroid hormone at the
level of the thyrotrope. When TRH (200 µg to 500 µg)
is administrated intravenously to normal subjects, a peak rise
in serum TSH occurs within 15 to 30 minutes. TSH response leads
to a rise in triiodothyronine after 90 to 150 minutes. Thyroxine
may increase somewhat later. Thyroid hormone inhibits pituitary
TSH secretion by direct negative feedback at the level of the pituitary,
but the effect on TRH stimulation has not been established. In
primary hypothyroidism, a TSH hyper-response characteristically
occurs. TRH is subject to rapid enzymatic breakdown in tissues
and body fluids.6
The thyroid has two main endocrine functions: secretion of the
thyroid hormones thyroxine (T4) and triiodothyronine
(T3) by the follicular cells and secretion of calcitonin
by the C cells.7 Thyroid hormones are first synthesized
as a prohormone, thyroglobulin, which is a large glycoprotein and within the colloid of the follicles.
This prohormone is also released by the thyroid and constitutes
a normal plasma component.7
The thyroid follicular cell traps iodide and uses it in the synthesis
of thyroid hormones. This metabolism proceeds by several well-defined
steps, as described by Van Herle et al.7
"(1) Iodide is trapped at the base of the cell by active transport
against an electrical gradient and is then transported to the follicular
lumen, where it is concentrated.
(2) Iodide reacts with a peroxidase, presumably at the interface
of the cell and the lumen, forming an oxidized species of iodine,
and is then incorporated into the tyrosyl groups of thyroglobulin in the colloid,
constituting monoiodotyrosine and diiodotyrosine residues in the
protein. . . .
(3) In the thyroglobulin molecule, already formed iodotyrosines
undergo oxidative coupling to form the iodothyronines T4 and
T3 and small amounts of reverse triiodothyronine (rT3).
This oxidation seems to be catalyzed by the same peroxidase."
"(4) By diffusion, . . . thyroglobulin slowly mixes around in the colloid;
iodination and oxidative coupling occur when the molecule encounters
the apex of the cells. . . .
(5) Secretion requires the incorporation of thyroglobulin into
colloid droplets for its digestion by lysosomal enzymes. Ingestion
may occur by two processes: micropinocytosis or macropinocytosis.
. . .
(6) Digestion of thyroglobulin is accomplished in secondary lysosomes,
first through reduction of disulfide bonds by glutathione and then
(7). . . after proteolysis, the released iodothyronines diffuse from the
secondary lysosomes in the cell and from the cell to the extracellular
space, . . . .
(8) Iodotyrosines are deiodinated in the cells by a NADPH+H+-dependent
deiodinase, and this iodine mixes with newly entered iodide. .
The metabolism of iodine in the thyroid is geared toward the
efficient use of a scarce and highly discontinuous supply of iodide
to the organism. The trapping of iodide can achieve a gradient
of more than 100 to 1. For physiologic concentrations, all the
iodide taken up is immediately oxidized and bound to thyroglobulin;
the thyroglobulin peroxidase system is able to synthesize iodothyronines
with only a few iodine atoms per thyroglobulin molecule. The high
storage capacity of thyroglobulin for iodine and of the colloid
lumen for thyroglobulin allows a normal thyroid to sustain a steady
secretion of iodothyronines for several weeks after a block of
As the key precursor in the biosynthesis of thyroid hormones,
thyroglobulin accounts for 75 percent of the total protein content
of the mammalian thyroid. It is a large globular glycoprotein with
a molecular weight of 660 000 and is composed of two polypeptide
Since the metabolism of the thyroid is geared toward the effective
use of a scarce supply of iodide to promote hormonal synthesis
and storage, it is not unexpected that certain mechanisms would
turn off this system when the availability of iodide is substantially
increased.7 Iodide in pharmacological doses decreases
thyroid blood flow, secretion, iodide trapping, protein iodination
and TSH (thyrotropin) activity.7
Thyroid function and growth are regulated primarily by the pituitary
gland through thyrotropin (TSH). Administration of thyrotropin (TSH) enhances
many steps of thyroid metabolism within a few minutes (e.g., iodide
binding to proteins, iodothyronine synthesis, thyroglobulin secretion
in the follicular lumen, colloid macropinocytosis and protein synthesis),
iodide trapping within a few hours and cell growth and multiplication
after a longer period of time.
In humans, TSH (thyrotropin) activates adenyl cyclase and enhances
cAMP accumulation in slices of thyroid. Proteins that bind TSH (thyrotropin)
with high affinity and adenylate cyclase have been demonstrated
on plasma membranes of the human thyroid. Four other physiologic
or pathologic extracellular signals activate thyroid adenylate
cyclase in the same manner as TSH (thyrotropin): prostaglandins E1 and
E2, adrenergic agents, thyroid-stimulating immunoglobulins and
human chorionic gonadotropin.7
The plasma of nearly all patients with Graves' disease (hyperthyroidism)
contains thyroid-stimulating immunoglobulins that have the following
properties: They enhance cAMP accumulation in human thyroid tissue
and activate human thyroid adenylate cyclase.7 They
also compete with TSH (thyrotropin) for receptors on human thyroid membranes.
No age-related variation of thyroglobulin concentration in man
has been reported. Some studies have reported higher values in
women than men. Moreover, the values are higher in pregnant women
at delivery than in nonpregnant controls.8
Autoimmunity substantially contributes to the pathogenesis of
a number of thyroid disorders, such as Hashimoto's thyroiditis,
primary myxedema, Graves' thyrotoxicosis and also Graves' ophthalmopathy.9 Ultimately,
the demonstration of cellular and humoral immune responses may
lead to the appropriate diagnosis.9 It has also been
shown that individuals with a family history of thyroid autoimmune
disease run an increased risk of acquiring such a syndrome.9 In
patients suffering from autoimmune-related thyroid diseases, one
frequently finds autoantibodies directed against thyroglobulin,
microsomal antigens (thyrosomal peroxidase [TPO]), and TSH receptors.
The microsomal antigen appears to be of particular importance for
the pathogenesis of these autoimmune processes. Autoantibodies
against TPO, a glycosylated integral membrane protein that is expressed
on the apical surface of thyroid epithelial cells, are closely
associated with the active phase of the disease and directly involved
in complement-mediated cytotoxicity.10
Numerous physician and professional organizations have recommended
testing for anti-TPO in cases of subclinical and/or symptomatic
hypothyroidism when TSH is elevated and free T4 is within
the normal reference interval.11 Anti-TPO antibodies
are an indicator of autoimmune thyroid disease, one of the most
common causes of hypothyroidism. When TSH was increased and anti-TPO
antibodies were present, the risk of overt clinical hypothyroidism
was 5 percent to 26 percent.11 The sensitivity and specificity
of any TPO assay is extremely dependent on the purity of the protein
used, i.e., above all, the stringent absence of thyroglobulin in
the antigen fraction. Currently, the use of recombinant TPO in
anti-TPO assays is highly recommended.10
Clinical Aspects Of Testing
Patients who have thyroid disease often present initially with
vague symptoms and nonspecific ailments, including (but not limited
to) malaise, weakness, fatigue and weight change, that are typical
manifestations. About 200 million people in the world have some
form of thyroid disease, according to the Web site of the Thyroid
Foundation of Canada (http://home.ican.net/~thyroid/Guides/HG00.html#1).
Elderly people, especially women, experience the highest incidence
of thyroid disease. Approximately 5 percent of all cases of mild
hypothyroidism develop into clinically significant thyroid failure.
Since thyroid function and thyroid hormone products interact as
part of a multiple-gland feedback loop, patterns of thyroid hormone
concentrations are often interpreted together to detect disease.
Generally, a specific combination of laboratory tests can aid in
the diagnosis of thyroid disease.
Secondary and/or tertiary hypothyroidism may sometimes be associated
with the production of biologically inactive TSH. Paradoxically
normal (or even elevated) serum TSH values may be seen in patients
with pituitary or hypothalamic hypothyroidism. This can probably
be explained by the reduced biologic activity of the secreted TSH.
Thyroxine (total T4) is the major hormone produced by the
thyroid gland. Total T4 concentrations have been readily
measured for years. However, of "total T4", 99.7 percent of blood-circulating total T4 is
bound to specific serum-binding proteins, and only the free hormone
is metabolically active. Any factor that alters serum-binding proteins,
particularly TBG, may affect the total T4 concentration
in the absence of thyroid dysfunction.12 Pregnancy and
estrogen-containing medications increase TBG concentrations; testosterone,
corticosteroids, severe illness, cirrhosis and nephrotic syndrome
lower TBG concentrations.
Direct free T4 (FT4) measurements can be
more reliable than total T4 measurements for clinical
purposes. Indirect measurements of FT4 do not yield
true FT4 results. One of the most widely used indirect
methods combines the measurement of T4 and thyroxine
hormone binding (usually T3 resin uptake [T3RU])
to calculate an estimated FT4 or FTI (free thyroxine
index). The method declines in diagnostic accuracy when patients
have abnormal binding proteins or there is binding protein impairment
associated with severe nonthyroidal illness.12 The inverse
relationship of the T3RU and TBG concentration is a
common source of confusion and a cause for misinterpretation of
thyroid function tests.12 Serum TSH measurement can
be mechanically viewed as an endogenous "free T4 sensor" that
will reflect FT4 status independent of binding protein
abnormalities. This concept, with the continued confusion surrounding
FTI methodology, has resulted in serum TSH measurement emerging
as the dominant thyroid function test.12 T3RU
tests are functionally and analytically obsolete with the availability
of good FT4 and free triiodothyronine (FT3)
automated assays.13 Currently, the recommended screening
test for hypothyroidism is the TSH, for follow-up FT4 can
be used to determine the severity of disease.10,12
The most recent testing algorithms for thyroid disease do not
suggest the use of calculated (or indirect) FT4 methods
(such as free thyroxine index), since more accurate and efficient
procedures are currently available.11,12 In 1998, the
American Medical Association (AMA) eliminated older automated chemistry
CPT codes and replaced them with organ or disease panels. The AMA
has modified these procedures each year since 1998. Two thyroid
test combinations, the thyroid panel (Total T4, T3 resin
uptake and a calculated FTI) and the thyroid panel with TSH, were
deleted by the AMA from the CPT handbook, Current Procedural
Terminology, for 2000. If a practitioner wishes to calculate
an FTI, the T3 uptake and the thyroxine must be ordered
Serum TSH provides the best biologic measurement of thyroid hormone
action in healthy ambulatory patients. The normal range for serum
TSH values varies approximately 10-fold from 0.4 to 4.5 mIU/mL,
and any value within this range is generally considered to reflect
a clinically euthyroid state. All individuals, however, appear
to possess their own unique TSH setpoint that may be located anywhere
in this range.
There is a prolonged lag phase for the full TSH response to T4 suppression.
It is generally documented that four to eight weeks are required
to achieve stable serum TSH concentrations following changes in
oral T4 dosage. Alterations in serum free thyroxine
concentrations produce logarithmically proportional responses in
pituitary TSH secretion. As an example, a twofold reduction in
serum free T4 concentrations nominally leads to an approximate
increase of 100-fold in serum TSH values. The administration of
pharmacologic doses of glucocorticoids or dopamine produces rapid
inhibition of TSH secretion and secondarily can cause substantial
lowering of serum TSH values.12
Determination of hyperthyroidism or thyrotoxicosis can be confirmed
by the combined finding of an abnormally high concentration of
serum thyroid hormones and, because of negative feedback inhibition
on the pituitary, a subnormal serum TSH level. Serum free T4 concentration
is increased in approximately 95 percent of ambulatory hyperthyroid
patients. An occasional hyperthyroid patient may have increased
T3 alone (T3 thyrotoxicosis). The newer sensitive
TSH assays clearly define a lower limit of the normal interval
for TSH, generally between 0.3 and 0.5 mIU/mL, varying according
to the individual assay. The serum TSH concentration in hyperthyroid
patients should clearly be subnormal (less than 0.1 mIU/mL). The
combination of an increase in serum free T4 concentration
and a decrease in serum TSH level to less than 0.1 mIU/mL suggests
the diagnosis of hyperthyroidism.1
The diagnosis of thyroid function can be difficult when patients
are taking medications that can alter the tests of thyroid function.
Phenytoin treatment of euthyroid patients results in a 30 percent
to 40 percent decrease in serum T4 and free T4 levels
and either normal or slightly decreased concentrations of T3 and
free T3. Treatment with carbamazepine or rifampin also
results in subnormal serum free T4 concentrations. Several
pharmacologic agents appear to act predominantly by decreasing
the rate of production of T3 from T4 in peripheral
tissues. These agents include glucocorticoids or propranolol hydrochloride
in high doses, oral cholecystographic radiopaque agents and amiodarone.12
Because of the ability of TSH measurements to reflect even a
minimal degree of thyroid dysfunction, it provides a highly accurate
method for excluding primary disease of the thyroid gland. The
finding of a normal serum TSH value in ambulatory patients virtually
excludes the diagnosis of thyroid disease.12 The approach
of using the TSH test first works better for ambulatory patients
than for hospitalized patients. Transiently elevated TSH values
may also be found in patients recovering from major physiologic
stress.15 Many clinical laboratories and major medical
centers have now moved to a TSH-centered or TSH-first approach
for thyroid disease diagnosis (Table 1). Reference laboratories
with large population bases have been able to develop accurate,
age-specific reference intervals that have proven useful in clinical
diagnosis (Tables 2 and 3).
Sensitive TSH is the initial test for evaluating suspected thyroid
disease. The majority of patients generally have normal TSH values
and no further testing is necessary. Values that are <0.1 µIU/mL
are suggestive of hyperthyroidism. For these patients, FT4 and
third- or fourth-generation TSH tests may be useful. As T3 toxicosis
is an occasional cause of hyperthyroidism, FT3 measurement
is suggested for patients with suppressed TSH and normal FT4.15
Today, in keeping with the goals of managed care, reflexive testing
and clinically relevant algorithms allow for optimal effectiveness.
By using a minimum number of thyroid tests, diagnostic accuracy
is not compromised and institutional cost-effectiveness is maintained.15
- Surks MI, Chopra IJ, Mariash CN, Nicoloff JT, Solomon DH. American
Thyroid Association guidelines for use of laboratory tests in
thyroid disorders. JAMA 1990; 263:1529-1532.
- Liu N, Garon J. A new generation of thyroid testing. ADVANCE
for Administrators of the Laboratory 1999; 11:29-30.
- Rafferty B, Gaines Das R. Comparison of pituitary and recombinant
human thyroid-stimulating hormone (rhTSH) in a multicenter collaborative
study: Establishment of the first World Health Organization reference
reagent for rhTSH. Clin Chem 1999; 45(12):2207-2208.
- Brent GA. The molecular basis of thyroid hormone action. N
Engl J Med 1994; 331(13):847-852.
- Sterling K. Thyroid hormone action at the cell level (First
of two parts). N Engl J Med 1979; 300(3):117-122.
- Jackson IMD. Thyrotropin-releasing hormone. N Engl J Med 1982;
- Van Herle AJ, Vassart G, Dumont JE. Control of thyroglobulin
synthesis and secretion (First of two parts). N Engl J Med 1979;
- Van Herle AJ, Vassart G, Dumont JE. Control of thyroglobulin
synthesis and secretion (Second of two parts). N Engl J Med 1979;
- Scherbaum WA. On the clinical importance of thyroid microsomal
and thyroglobulin antibody determination. Acta Endocrinol
(Copenh) 1987; Suppl 281:325-329.
- Tri-Delta Diagnostics. TPO Antibodies: Quantitative Enzyme
Immuno Assay for the Determination of Thyroid Peroxidase Antibodies
in Serum or Plasma Samples. Osceola, WI: Elias USA; 1992.
- Whiteside-Yim C, MacAdams MR. Thyroid disorders: The general
internist's approach. Postgraduate Med 1987; 81(5):231-245.
- Spencer CA, Nicoloff JT. Serum TSH measurement: A 1990 status
report. Thyroid Today 1990; 13(4):1-12.
- Johnson GF. Laboratory Diagnosis and Screening of Thyroid
Disease 1996; September 24:1-16. [ASCP teleconference series].
- Feldkamp CS. Thyroid testing algorithms: A rational design
can improve patient care and reduce costs. Clin Laboratory
- Klee GG, Hay ID. Role of thyrotropin measurements in the diagnosis
and management of thyroid disease. Clin Lab Med 1993;
- Endo-text on-line endocrinology
textbook has testing protocols.
tests Practice Guidelines, National Academy of Clinical
Biochemistry (NACB), USA.
- Soule JB, "Case History: Thyrotoxicosis Factitia Secondary to Triiodothyronine Acquired Electronically From Abroad", JSCMA 103(3):60-62, April 2007.
- Fatourechi V, "Subclinical Hypothyroidism: An Update for Primary Care Physicians", Mayo Clinic Proceedings 84(1):65-70, January 2009.
(posted 2001; latest addition 13 March 2010)