Endocrinology‎ > ‎Thyroid truths‎ > ‎

Diagnostic assumptions for TSH and Free T4

The TSH testing is accurate only when 6 major assumptions are met.  1) Steady state conditions  2) Normal TSH-to-target organ hormone relationship  3) Tissue responsiveness proportionality to hormone concentration 4) The TSH assay measure active hormone  5) The assay can reliably distinguish low from normal values 6) The reference ranges are appropriate. Your endocrinologist at Houston Thyroid and Endocrine can help you fully understand these details. This is a reference section only. 

 1) Steady state conditions:

This first assumption should be questioned whenever an irregular result occurs during an associated illness or with medications that affect the pituitary-thyroid axis. The half life (how long it takes half the substance to dissappear) of plasma TSH is approximately 1 hour  and the half life of plasma T4  is approximately 1 week.  These half-lives differ so much that sudden/recent changes of the pituitary-thyroid axis will often result in a temporary non steady-state condition. With its much shorter half-life, serum TSH deviates more rapidly from the steady state. Other common deviations from the steady state: short-term pulsatile or diurnal fluctuations in hormone secretion, responses to treatment with thyroid hormone, and spontaneous evolution of disease such as subacute thyroiditis or postpartum thyroid dysfunction.

Early treatment with antithyroid drugs (1)

Early response to T4 therapy

Evolution of transient thyroid dysfunction

Recovery from severe illness (2)


2) Normal TSH-to-target organ hormone relationship

During the intial treatment of a patient who has had a perior of prolonged hyperthyroidism, the TSH may remain low for several months after serum free T4 becomes normal (4) ).  Also, after severe prolonged duration of hypothyroidism, or in some children treated for congenital hypothyroidism (3), the TSH may remain elevated  despite normalization of serum T4. Serum TSH will then give an inaccurate indication of thyroid status and so with the potential for over-treatment if this parameter alone is used to assess therapy.

Alternative thyroid stimulators

Immunoglobulins (5)

Chorionic gonadotrophin (6)

Medications that influence TSH secretion

T3 , triiodothyroacetic acid (7)

Glucocorticoids (8)

Dopamine (9)

Amiodarone (10)

Recent hyperthyroidism (4)

Recent longstanding hypothyroidism

Treated congenital hypothyroidism (11)

TSH receptor mutations (12)

Variable individual setpoint (13-17)


3) Tissue responsiveness proportionality to hormone concentration

The active or free concentrations of T3 and T4 generally correlate well with clinical features.  In the rare condition called generalized thyroid hormone resistance, high serum free T3 and T4 concentrations are "normal"  when the patient is in thyroid hormone balance.   The onset and offset of thyroid hormone action is slow, so that tissue responses may lag behind changes in serum concentrations of free T4 and T3.  Since there is a lack of convenient, sensitive, specific, objective measurements of thyroid hormone action in these patients,  assessment remains predominantly clinical. There are other tests that can help manage these types of patients including  measurements of oxygen consumption (18), sex hormone binding globulin (19), angiotensin converting enzyme (20) and serum ferritin (21), as well as several indices of cardiac contractility.

Hormone resistance syndromes (22)

Slow onset/offset of thyroid hormone action

Drug effects

Amiodarone ( 23)

Phenytoin (24)


4) The TSH assay measures active hormone

TSH and iodothyronine assays make comparative, rather than absolute, measurements of hormone concentrations, based on the premise that samples and assay standards differ only in their concentration of analyte. This assumption is no longer applicable in these situations:  dissimilar protein binding of tracer (25), the presence of binding competitors (26,27), or possible nonspecific interference with enzymatic, fluorescent, or chemiluminescent detection systems. Circulating T3 and T4 autoantibodies may invalidate immunoassays by sequestering the assay tracer (180), while heterophile mouse or sheep antibodies and rheumatoid factor can interfere with immunoglobulin aggregation, or with cross linking of the signal and capture antibodies (28,29).  In hypothalamic hypothyroidism, the secreted TSH has decreased bioactivity (30), whereas activity may be enhanced in thyroid hormone resistance, primary hypothyroidism and in some TSH-producing tumors (31).

Unmeasured agonist in excess ( T3, Triac, hCG)

TSH of altered biologic activity (32)

Spurious immunoassay results


Heterophilic antibodies (33)

Free T4

Abnormal serum binding proteins (34)

Autoantibodies (35)

Heterophilic antibodies (36)

Medications that inhibit protein binding (26)

Heparin artefact (37)


5) The assay can reliably distinguish low from normal values

With the use of third generation TSH assays, this assumption is always met.


6) The reference ranges are appropriate

TSH values as well as the T4 and T3 values can have different normal ranges based upon patient age, pregnancy status, and nutritional status/illness. (38)




1) Mariotti S, Caturegli P, Piccolo P, et al. Antithyroid peroxidase autoantibodies in thyroid diseases. J Clin Endocrinol Metab 1990;71:661-9.

2) Després N, Grant AM. Antibody interference in thyroid assays: a potential for clincial misinformation. Clin Chem 1998;44:440-54.

3) Mazzaferri EL, Kloos RT. Current approaches to primary therapy for papillary and follicular thyroid cancer. J Clin Endocr Metab 2001; 86: 1447-63.

4) Mariotti S, Caturegli P, Piccolo P, et al. Antithyroid peroxidase autoantibodies in thyroid diseases. J Clin Endocrinol Metab 1990;71:661-9.

5) Lazarus JH The effects of lithium therapy on thyroid and thyrotropin-releasing hormone. Thyroid 1998; 8; 909-13.

6) Murakami T, Masumoto T, Michitaka K, et al. Prediction of interferon–alpha-induced thyroid dysfunction in patients with chronic hepatitis C. J Gastro Hepat 1995; 10: 528-31.

7) Lazarus JH, Hall R, Othman S, et al. The clinical spectrum of postpartum thyroid disease. Q J Med 1996; 89: 429-35

8) Pop VJ, Maartens LH, Leusink G, et al. Are autoimmune thyroid dysfunction and depression related ? J Clin Endocrinol Metab 1998;83:3194-7.

9)Costagliola S, Morgenthaler NG, Hoermann R, et al. Second generation assay for thyrotropin receptor antibodies has superior diagnostic sensitivity for Graves' disease J Clin Endocrinol Metab 1999; 84:90-7.

10) Jarlov AE, Nygaard B, Hegedus L et al. Observer variation in the clinical and laboratory evaluation of patients with thyroid dysfunction and goiter. Thyroid 1998;8:393-8.

11)  Laurberg P, Nygaard B, Glinoer D, et al. Guidelines for TSH-receptor antibody measurements in pregnancy: results of an evidence-based symposium organized by the European Thyroid Association. Europ J Endocrinol 1998;139:584-6.

12)Michelangeli V, Poon C, Taft J, et al. The prognostic value of thyrotropin receptor antibodymeasurement in the early stages of treatment of Graves' disease with antithyroid drugs. Thyroid 1998; 8: 119-24

13) Docter R, Krenning EP, de Jong M, et al. The sick euthyroid syndrome: changes in thyroid hormone serum parameters and thyroid hormone metabolism. Clin Endocrinol 1993; 39:499-518.

14) Spencer CA, Schwarzbein D, Guttler RB, et al. Thyrotropin (TSH)-releasing hormone stimulation test responses employing third and fourth generation TSH assays. J Clin Endocrinol Metab 1993;76:494-8

15) Laurberg P. Persistent problems with the specificity of immunometric TSH assays. Thyroid 1993; 3: 279-83.

16)  Morley JE, Neuroendocrine control of thyrotropin secretion. Endocr Rev 1981; 2: 396-436.

17)  Staub J-J, Noelpp B, Grani R, et al. The relationship of serum thyrotropin (TSH) to the thyroid hormones after oral TSH-releasing hormone in patients with preclinical hypothyroidism. J Clin Endocrinol Metab 1983; 56:449-53.

18)  Roti E, Minelli R, Gardini E, et al. The use and misuse of thyroid hormone. Endocr Rev 1993; 14: 401-23.

19) DeGroot LG Dangerous dogmas in medicine: the non-thyroidal illness syndrome. J Clin Endocrinol Metab 1999; 84:151-64

20) Sapin R, Schlienger J-L, Gasser F et al: Intermethod discordant free thyroxine measurements in bone marrow-transplanted patients. Clin Chem 2000;46:418-22.

21) Van den Berghe G. Novel insights into the neuroendocrinology of critical illness. Eur J Endocrinol 2000; 143;1-13.

22) Burmeister LA, Reverse T3 does not reliably differentiate hypothyroid sick syndrome from euthyroid sick syndrome. Thyroid 1995; 5: 435-41

23) Jarlov AE, Nygaard B, Hegedus L et al. Observer variation in the clinical and laboratory evaluation of patients with thyroid dysfunction and goiter. Thyroid 1998;8:393-8.

24) Hollander CS, Mitsuma T,Nihei N, et al Clinical and laboratory observations in cases of triiodothyronine toxicosis confirmed by radioimmunoassay. Lancet 1970; 1: 609-11

25) Van den Berghe G, Wouters P, Weekers F et al: Reactivation of pituitary hormone release and metabolic improvement by infusion of growth hormone releasing peptides and thyrotropin releasing hormone in patients with protracted critical illness. J Clin Endocrinol Metab 1999; 84: 1311-23

26)  Rothwell PM, Udwadia ZF, Lawler PG. Thyrotropin concentration predicts outcome in critical illness. Anaesthesia 1993;48:373-6.

27 )Stevenson HP, Archbold GPR, Johnston P, et al Misleading serum free thyroxine results during low molecular weight heparin treatment. Clin Chem 1998;44;1002-7.

28) Spencer CA, LoPresti JS, Patel A, et al. Applications of a new chemiluminometric thyrotropin assay to subnormal measurement. J Clin Endocrinol Metab 1990;70:453-60.

29)Franklyn JA, Black EG, Betteridge J et al. Comparison of second and third generation methods for measuring serum thyrotropin in patients receiving thyroxine and in those with nonthyroidal illness. J Clin Endocrinol Metab 1994; 78: 1368-71.

30 )Lum SMC, Kaptein EM, Nicoloff JT Influence of nonthyroidal illnesses on serum thyroid hormone indices in hyperthyroidism. West J Med 1983;128:670-3.

31)  Ladenson PW, Braverman LE, Mazzaferri EL, et al. Comparison of administration of recombinant human thyrotropin with withdrawal of thyroid hormone for radioactive iodine scanning in patients with thyroid carcinoma. N Engl J Med 1997; 337: 888-96.

32) Sterling K, Refetoff S, Selenkow H. T3 thyrotoxicosis. Thyrotoxicosis due to elevated serum triiodothyronine levels. JAMA 1970; 213:571-5.

33)Inada M, Sterling K: Thyroxine transport in thyrotoxicosis and hypothyroidism J Clin Invest 1967;46:1442-50.

34) Nauman JA, Nauman A, Werner SC: Total and free triiodothyronine in human serum. J Clin Invest 1967;46:1346-55.

35) Homsanit M, Sriussadaporn S, Vannasaeng et al Efficacy of single daily dosage of methimazole vs. propylthiouracil in the induction of euthyroidism. Clin Endocrinol 2001;545:385-90.

36) Davies PH, Franklyn JA, Daykin J et al. The significance of TSH values measured in a sensitive assay in the follow-up of hyperthyroid partients treated with radioiodine. J Clin Endocrinol Metab 1992; 74: 1189-94.

37 ) Fraser WD, Biggart EM, O'Reilly DStJ, et al. Are biochemical tests of thyroid function of any value in monitoring patients receiving thyroxine replacement? Br Med J 1986;293:808-10