Laboratory evaluation of thyroid function

Laboratory evaluation of thyroid function

Blood tests can detect thyroid dysfunction, which can result in cardiac, GI, and menstrual disturbances as well as abnormalities in fetal neural development.

B lood tests to measure thyroid function are readily available and widely used.To un- derstand a test’s scientific basis and what it can tell us, a quick review of the thyroid gland’s pathophysiology is in order.The major hormone secreted by the thyroid is thyroxine,also called T ₄ because it contains four iodine atoms. ¹ To exert its effects, T ₄ is converted to triiodothyronine (T 3 ) by the removal of an iodine atom.This oc- curs mainly in the liver and in certain tissues where T 3 acts,such as the brain.The amount of T 4

produced by the thyroid is controlled by thyroid- stimulating hormone (TSH), which is produced and released by the pituitary gland.As is the case with many endocrine glands, regulation of the thyroid occurs through a negative feedback loop.

If the pituitary detects very little T 4 in the blood, it produces more TSH, which then signals the thyroid to produce more T 4 . Once the T 4 in the bloodstream rises above a certain level, the pitu- itary’s production of TSH is shut off,thereby sig- naling the thyroid to produce less T ₄ . Conditions that interfere with this normal process are cate- gorized as influencing the thyroid either directly or indirectly. Whichever the case, simple blood tests are useful in identifying the most common causes of thyroid dysfunction.

Evaluating thyroid function

The serum TSH is the best initial test of thyroid function. The latest generation of this assay has high sensitivity and is an excellent screening tool for those patients with a low pretest probability of thyroid disease. ^2,3 A TSH of 0.5-4.0 mU/L is

FEATURE: LAURA M. GUNDER, DHSC, MHE, PA-C, AND SARA HADDOW, MSA, PA-C

© ISM / PHOTOTAKE

Myxedema is a skin condition caused by the deposition of hyaluronic acid in

patients with

thyroid disease.

thyroid function than FT 4 . ⁴ Because the FTI corrects for changes in TBGs, it can be used to diagnose thyroid disorders in patients with protein abnormalities and to monitor their therapy. For example, women who are pregnant have in- creased globulin levels, while persons on certain globulin- binding drugs, e.g., phenytoin (Dilantin), may have decreased levels of available globulin.

An elevated FT ₄ or FTI indicates hyperthyroidism, while a low FT 4 or FTI indicates hypothyroidism. ^1,4 Combining the TSH test with the FT ₄ or FTI accurately determines how the thyroid is functioning.The finding of an elevated TSH and low FT ₄ or FTI indicates primary hypothyroidism due to disease in the thyroid itself. ^1,4 A low TSH and low FT 4 or FTI indicates secondary hypothyroidism, i.e., a problem outside the thyroid, likely involving the pituitary. ^1,4 A low TSH with an elevated FT ₄ or FTI is found in individuals who have hyperthyroidism. ^1,4 (Table 1 summarizes the interpretation of various test results.) highly diagnostic for normal thyroid function. A high TSH

(>5.0 mU/L is an indication for further testing, such as a free T 4 (FT 4 ) determination or a free thyroxine index (FTI).

When there is a high pretest probability for thyroid disease, e.g., in the presence of risk factors or clinical signs and symp- toms, initial testing should include a serum TSH as well as an FT 4 or an FTI. ^2,3 A patient who has a TSH in the gray zone (4.1–5.0 mU/L) is very likely to develop hypothyroidism and should be screened regularly.Treatment for subclinical hypo- thyroidism in asymptomatic individuals with TSH <10 mU/L is controversial. ²

A high TSH indicates that the thyroid is failing because of a problem directly affecting the gland. ¹ This direct relationship is known as primary hypothyroidism. Occasionally, a low TSH may result from an abnormality in the pituitary that prevents it from making enough TSH to stimulate the thy- roid.This indirectly caused state is known as secondary hy- pothyroidism. The opposite situation, in which the TSH level is low,usually indicates that the person has an overactive thyroid that is producing too much thyroid hormone (hy- perthyroidism). ¹ In most healthy individuals, a normal TSH value means that the thyroid is functioning well and the pa- tient’s condition is considered to be euthyroid.The newest version of the TSH assay is sensitive enough to distinguish hyperthyroidism from the below-normal TSH values ob- served in transient circumstances (such as euthyroid sick syndrome). ^2-4 The TSH is likewise useful for following pa- tients on thyroid medication. ^2-4

Generally, the serum T ₄ represents about 90% of circulating thyroid hormone. ⁴ T 4 circulates in the blood in two forms:T 4

bound to proteins which prevent the hormone from entering the various tissues that need it and FT 4 (not bound to protein), which enters the various target tissues and exerts its effects.

The FT 4 fraction represents only about 5% of total T 4 but is the most important for determining how the thyroid is func- tioning since it is the metabolically active form of the hor- mone. ⁴ Abnormal protein levels can have significant effect on the total T ₄ results. ⁴ For example, an increase in thyroxine- binding globulins (TBGs) will raise the level of total T 4 , while a decrease in TBG will lower total T ₄ . ⁴ Note that while changes in TBGs,which transport T 4 and T 3 ,can affect the lev- els of circulating T ₄ , such alterations may not affect the pa- tient’s metabolic state.

Variations among laboratory test methods and variance in patients’globulin status make the FTI a better indicator of true

TABLE 1. Thyroid function test interpretation

TSH result Subsequent

FT

result* Possible diagnoses Elevated TSH

(>5 mU/L) Low FT

Primary hypothyroidism

Normal FT

Subclinical hypothyroidism

High FT

TSH-mediated hyperthyroidism (secondary or tertiary hyperthy- roidism)

Low TSH

(<0.1 mU/L) Low FT

Central hypothyroidism (rare)

Normal FT

Subclinical hyperthyroidism Check T

; may recheck FT

and T

every two to three months

High FT

Hyperthyroidism or thyrotoxicosis Check RAIU to identify cause

*In some patients, an freethyroxine index (FTI) may provide more information. See text for discussion of FTI.

FT

=free thyroxine, RAIU=radioactive iodine uptake;TSH=thyroid-stimulating hormone

Sources: Baskin HJ et al

;Wilson GR and Curry RW

; Demers LM, Spencer CA. Laboratory

Clinical Chemistry; 2002. Available at www.aacc.org/members/nacb/Archive/LMPG /ThyroidDisease/Pages/ThyroidDiseasePDF.aspx. Accessed October 26, 2009; Supit EJ, Peiris AN. Interpretation of laboratory thyroid function tests for the primary care physi- cian. South Med J. 2002;95:481-485.

A high TSH indicates that the thyroid is failing because of a problem directly affecting the gland.

This is known as primary hypothyroidism.

Continues on page 30

patient with clinical hyperthyroidism, suspect autoimmune thyroid disease. ^1,4

A summary of the tests used to evaluate thyroid function appears in Table 2.

Which tests to order and when

In clinical practice, three basic scenarios indicate a need for laboratory evaluation of thyroid function: (1) suspicion of thyroid disease based on clinical signs and symptoms, ^1-4 (2) screening for thyroid disease, ^1-6 and (3) evaluation of treat- ment for thyroid disease. ^1,4,7,8

Working up symptomatic patients When clinical signs and symptoms of hypothyroidism or hyperthyroidism (Table 3) are present, evaluation of a serum TSH and FTI or FT ₄ is indicated. ^1,4 Because thyroid dysfunction may develop insidi- ously over a long period, consideration of subclinical thyroid T 3 tests are often useful to diagnosis hyperthyroidism or to

determine its severity.Patients who are hyperthyroid will have an elevated T 3 level.In some patients with a low TSH,only the T ₃ is elevated and the FT ₄ or FTI is normal. ^1,4 T ₃ testing rarely is helpful in the hypothyroid patient, since it is the last test to become abnormal. ^1,4 Clinically, this raises the possibility for patients to be severely hypothyroid with a high TSH, low FT 4

or FTI,and a normal T ₃ .

Some persons produce antibodies against their thyroid that either stimulate or damage the gland. The two major anti- bodies that interfere with thyroid function are antithyroid peroxidase (anti-TPO) and antithyroglobulin. ^1,4 Both anti- bodies are readily detected in the serum. The presence of anti-TPO and/or antithyroglobulin antibodies in a patient with clinical hypothyroidism is diagnostic for Hashimoto’s thyroiditis. ^1,4 When these same antibodies are detected in a

Consideration of subclinical thyroid disorders is crucial in the presence of abnormal test results regardless of clinical presentation.

THYROID FUNCTION

Entity

tested Description Clinical utility

TSH Thyroid-stimulating hormone or thyrotropin • Best thyroid function screening test

• Initial test for suspected thyroid disease

• Used to follow patients on thyroid hormone therapy

• Used in conjunction with T

to manage patients with Graves’ disease

T

Serum total thyroxine • Used to make diagnosis of underactive or overactive thyroid when TSH is abnormal

• Used with TSH for monitoring patients with Graves’ disease

• Newborn screening test for hypothyroidism

• Fairly accurate in patients with no protein abnormalities and not pregnant

FT

Free thyroxine is the metabolically active thyroid hormone – not bound to protein

• Should be ordered when TSH is abnormal to determine thyroid hyperfunction or hypofunction.

FTI

Free thyroxine index – measure of free T

determined by measuring thyroxine level and either thyroid- binding globulin or hormone-binding ratio

• Used for making the diagnosis of thyroid disease in patients with protein abnormalities and in pregnant patients

• Used for monitoring therapy in above patient groups with hyperthyroidism

T

Serum total triiodothyronine • Used to diagnose hyperthyroidism when TSH is low and T

is still normal

Thyroid antibodies

• Antithyroid peroxidase (antimicrosomal) antibodies

• Antithyroglobulin antibodies

• Used to diagnose suspected Hashimoto’s thyroiditis in hypothyroidism

• Used to diagnose autoimmune thyroiditis or Graves’ disease in hyperthyroidism

Sources: Baskin HJ et al

;Wilson GR and Curry RW

; Demers LM, Spencer CA. Laboratory Support for the Diagnosis and Monitoring of Thyroid Disease. American Association for Clinical Chemistry; 2002. Available at www.aacc.org/members/nacb/Archive/LMPG/ThyroidDisease/Pages/ThyroidDiseasePDF.aspx. Accessed October 26, 2009; Supit EJ, Peiris AN. Interpretation of laboratory thyroid function tests for the primary care physician. South Med J. 2002;95:481-485.

TABLE 2. Summary of blood tests to evaluate thyroid function and their clinical utility

Hypothyroid patients who are started on levothyroxine should have their TSH measured every six to eight weeks to guide dose adjustments. ^2,4 Dosing is considered therapeutic once TSH levels reach normal ranges and the patient is no longer symptomatic. ^1-4

Female patients who become pregnant while taking levothyroxine should have a TSH level assessed immediately after pregnancy is diagnosed, since the replacement dose of levothyroxine will typically increase during pregnancy. ^1-4 These patients will also need TSH assessment at regular inter- gardless of clinical presentation. Subclinical hyperthyroidism

and subclinical hypothyroidism are exclusively laboratory di- agnoses. ^7,8 Subclinical hypothyroidism should be suspected when the serum TSH is increased above the upper limit of the reference range (>5.0 mU/L) in combination with a normal T ₄ . ^1,5,7,8 Conversely,subclinical hyperthyroidism is likely when TSH is decreased below the lower limit of the reference range (<0.10 mU/L) in the presence of a normal T ₄ (Table 1). ^1,5,7,8

Screening Patients not previously diagnosed or treated for thyroid disease should be screened if they are older than 60 years or if they have a personal history of surgery or irradiation of the thyroid or neck, any family history of autoimmune disease, or an existing thyroid nodule or goiter. ^3,6 Screening is also indi- cated for those patients who are currently using or who have a history of long-term use of amiodarone or lithium. ^3,6 New- borns are screened to detect hypothyroidism in infancy by per- forming a serum T ₄ level on the blood spot collected shortly after birth; hypothyroidism that is detected early can be treated and mental retardation or cretinism prevented. ^2-4

Subclinical hyperthyroidism is estimated to occur in 2% of the adult population. ^1,5,7,8 The condition may be due to TSH suppression from an exogenous source or to endogenous pro- duction of thyroid hormone that suppresses pituitary TSH production and keeps FT 4 and T 3 levels normal. ^1,2,7,8 Such cir- cumstances may represent the early stages of clinical hyper- thyroidism and should be considered a risk factor for the development of osteoporosis and adverse cardiac manifesta- tions, such as atrial fibrillation. ^1,2 Once the suppressed TSH is detected, repeat evaluation is needed to document that the low level is persistent.The American Academy of Clinical En- docrinologists (AACE) recommends that TSH, FT ₄ , and T ₃ determinations be repeated two to four months after the ini- tial discovery of low TSH. ^1,2 While treatment guidelines for subclinical hyperthyroidism have not been established, pa- tients who have persistently low TSH levels should be re- evaluated at six-month intervals thereafter. ¹

Subclinical hypothyroidism occurs in about 5% of the adult population, but prevalence may be as high as 20% in women older than 60 years. ^1,5,7,8 Approximately 5% of patients with subclinical hypothyroidism will progress to clinical hypothy- roidism each year. ^5,8 Subclinical hypothyroidism increases the risks for hyperlipidemia, atherosclerosis, and possibly neu- robehavioral disorders. ^2,5,7,8 Patients with subclinical hypothy- roidism (TSH >5.0 mU/L) should be re-evaluated within three months and then every six months. ⁸

Treatment monitoring The same tests that are used for diagnosis of thyroid disease can be used to follow treatment.

Hypothyroidism Hyperthyroidism

• Cold intolerance

• Fatigue

• Depression

• Memory impairment/

decreased concentration

• Weight gain

• Dry skin and dry hair

• Hair loss with increasing coarseness

• Constipation

• Myalgias

• Menstrual irregularities

• Hoarseness

• Goiter

• Bradycardia

• Myxedema

• Hyperlipidemia

• Delayed return of deep tendon reflexes

• Heat intolerance

• Muscle weakness

• Fine resting tremor

• Tachycardia

• Palpitations/

irregular heart rate

• Fatigue

• Weight change

• Increased frequency of stool

• Irritability

• Anxiety

• Sleep disturbance

• Ophthalmopathy

• Menstrual irregularities

• Myxedema

• Hyperreflexia

Sources: Baskin HJ et al

;Wilson GR and Curry RW

; Fitzgerald PA. Endocrine disorders. In: McPhee SJ, Papdakis MA, eds. Current Medical Diagnosis and Treatment.

48

ed. New York, NY: McGraw-Hill; 2009:976-1003.

AT A GLANCE

● The serum thyroid-stimulating hormone is the best initial test of thyroid function.

● Abnormal protein levels can have significant effect on the total thyroxine (T

) results.

● Subclinical hyperthyroidism and subclinical hypothyroidism are exclusively laboratory diagnoses.

● Re-evaluate patients with subclinical hypothyroidism within

three months of detection and then every six months.

vals throughout the pregnancy and postpartum period even if they had stable TSH levels prior to pregnancy. ^1-4 Left un- treated,maternal hypothyroidism can cause defects of the fetal neural development.

Patients with low TSH who are treated for Graves’ disease, thyroid nodules, and thyroiditis may also be monitored using TSH and T ₄ levels at four-week intervals during treatment. ^1-4 Monitoring of such patients should continue until thyroid levels normalize and symptoms resolve. ■

Dr. Gunder and Ms. Haddow are assistant professors in the School of Allied Health Sciences at the Medical College of Georgia in Augusta.

References

1. Ladenson P, Kim M.The thyroid. In: Goldman L,Ausiello D, eds. Cecil Medi- cine. 23rd ed. Philadelphia, Pa.: Saunders; 2007: chap 244.

2. Baskin HJ, Cobin RH, Duick DS, et al;American Association of Clinical Endocrinologists Thyroid Task Force.American Association of Clinical Endocrinologists medical guidelines for clinical practice for the evaluation and treatment of hyperthyroidism and hypothyroidism. Endocr Pract. 2002;

8:457-469.

3. American Academy of Family Physicians (AAFP). Summary of recommen- dations for clinical preventive services. Revision 6.8. Leawood, Kan.:Ameri- can Academy of Family Physicians (AAFP); October 2009.Available at www.aafp.org/online/etc/medialib/aafp_org/documents/clinical/CPS /rcps08-2005.Par.0001.File.tmp/Oct2009RCPSwithedits.pdf.

4. Wu A, ed. Teitz Clinical Guide to Laboratory Tests. 4

ed. Philadelphia, Pa.:

Saunders; 2006.

5. U.S. Preventive Services Task Force. Screening for thyroid disease: recom- mendation statement. Ann Intern Med. 2004;140:125-127.Available at www.annals.org/cgi/reprint/140/2/125.pdf.

6. Vanderpump MP,Tunbridge WM, French JM, et al.The incidence of thyroid disorders in the community: a twenty-year follow up of the Wickham Sur- vey. Clin Endocrinol (Oxf). 1995;43:55-68.

7. Surks MI, Ortiz E, Daniels GH, et al. Subclinical thyroid disease: scientific review and guidelines for diagnosis and management. JAMA. 2004;291:228- 238.Available at jama.ama-assn.org/cgi/content/full/291/2/228.

8. Wilson GR, Curry RW. Subclinical thyroid disease. Am Fam Physician. 2005;

72:1517-1524.Available at www.aafp.org/afp/20051015/1517.html.

All electronic documents accessed November 10, 2009.

THYROID FUNCTION

What do you think?

Add your comments to this article — or any article — by going to

www.clinicaladvisor.com.You will also see what your colleagues are saying.

Click Here

“We need a better piling system.”

w Yorker Collection 2009 from cartoonbank.com.All Rights Reserved.

“I wasn’t texting.I was building this ship in a bottle.”

“You smell like a chimney.”