Introduction

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;
• elderly 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.

Biochemistry

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

Physiology 

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 by proteolysis.

(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. . . ."⁷

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 synthesis.⁷

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 subunits.⁷

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 separately.¹⁴

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).

Conclusion

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

References:

1. 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.
2. Liu N, Garon J. A new generation of thyroid testing. ADVANCE for Administrators of the Laboratory 1999; 11:29-30.
3. 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.
4. Brent GA. The molecular basis of thyroid hormone action. N Engl J Med 1994; 331(13):847-852.
5. Sterling K. Thyroid hormone action at the cell level (First of two parts). N Engl J Med 1979; 300(3):117-122.
6. Jackson IMD. Thyrotropin-releasing hormone. N Engl J Med 1982; 306(3):145-154.
7. Van Herle AJ, Vassart G, Dumont JE. Control of thyroglobulin synthesis and secretion (First of two parts). N Engl J Med 1979; 301(5):239-246.
8. Van Herle AJ, Vassart G, Dumont JE. Control of thyroglobulin synthesis and secretion (Second of two parts). N Engl J Med 1979; 301(6):307-314.
9. Scherbaum WA. On the clinical importance of thyroid microsomal and thyroglobulin antibody determination. Acta Endocrinol (Copenh) 1987; Suppl 281:325-329.
10. 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.
11. Whiteside-Yim C, MacAdams MR. Thyroid disorders: The general internist's approach. Postgraduate Med 1987; 81(5):231-245.
12. Spencer CA, Nicoloff JT. Serum TSH measurement: A 1990 status report. Thyroid Today 1990; 13(4):1-12.
13. Johnson GF. Laboratory Diagnosis and Screening of Thyroid Disease 1996; September 24:1-16. [ASCP teleconference series].
14. Feldkamp CS. Thyroid testing algorithms: A rational design can improve patient care and reduce costs. Clin Laboratory News 1997;10:6-8.
15. Klee GG, Hay ID. Role of thyrotropin measurements in the diagnosis and management of thyroid disease. Clin Lab Med 1993; 13(3):673-682.
16. Endo-text on-line endocrinology textbook has testing protocols.
17. thyroid tests Practice Guidelines, National Academy of Clinical Biochemistry (NACB), USA.
18. Soule JB, "Case History: Thyrotoxicosis Factitia Secondary to Triiodothyronine Acquired Electronically From Abroad", JSCMA 103(3):60-62, April 2007.
19. 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)