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.
6 Major Assumptions
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
Chorionic gonadotrophin (6)
Medications that influence TSH secretion
T3 , triiodothyroacetic acid (7)
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
Amiodarone ( 23)
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)
Abnormal serum binding proteins (34)
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)
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