Thyroid Function Tests and Interpretation

Normal Reference Ranges

• The American Association of Clinical Endocrinologists recommends that normal thyroid lab values are TSH 0.45-4.5 mIU/L, free T4 within reference range, and free T3 within reference range 1, 2
• TSH reference range is 0.45-4.5 mIU/L, based on NHANES III data from a disease-free population 1, 2, 3

Lab Patterns in Hyperthyroidism

• In hyperthyroidism, TSH is suppressed below 0.45 mIU/L while free T4 and/or free T3 are elevated or normal 1, 4
• TSH is often undetectable (<0.01 mIU/L) in overt hyperthyroidism 4
• In subclinical hyperthyroidism, TSH is low while free T4 and T3 remain normal 1, 4

Lab Patterns in Hypothyroidism

• In hypothyroidism, TSH is elevated above 4.5 mIU/L while free T4 is low or normal 1, 2
• Approximately 75% of patients with elevated TSH have values <10 mIU/L 4
• In subclinical hypothyroidism, TSH is elevated while free T4 remains normal 1, 2

Diagnostic Algorithm

• If TSH is elevated (>4.5 mIU/L) with low free T4, it indicates overt primary hypothyroidism 1
• If TSH is suppressed (<0.45 mIU/L) with elevated free T4 and/or free T3, it indicates overt hyperthyroidism 1, 5

Common Pitfalls and Caveats

• Non-thyroidal illness can cause low TSH, low T3, and normal/low T4 without actual thyroid dysfunction 1, 4
• Medications such as dopamine, glucocorticoids, and dobutamine can suppress TSH without causing hyperthyroidism 4
• Pregnancy can cause physiologically low TSH with normal free hormone levels 4
• Central hypothyroidism is characterized by low/normal TSH with low free T4 due to pituitary or hypothalamic dysfunction 2

Thyroid Function Tests

Factors Affecting T4 Levels

• Iodine from CT contrast can transiently impact thyroid function tests, as reported by the Annals of Oncology 6

Medications and Iodine Exposure

• The exposure to iodine from CT contrast can affect thyroid function tests, according to a study published in 2017 by the Annals of Oncology 6

Thyroid Function Test Interpretation

Diagnostic Criteria

• Values below 0.1 mIU/L are generally considered low, while values above 6.5 mIU/L are considered elevated in screening contexts, according to the American Medical Association, as reported in the journal formerly known as the journal of the American Medical Association 7
• If TSH >4.5 mIU/L with normal free T4, it indicates subclinical hypothyroidism, as stated by the American Medical Association, with a strength of evidence based on a 2004 study 8

Quality Control and Assay Considerations

• The TSH assay used should have a functional sensitivity of at least 0.02 mIU/L, independently established by each laboratory, as recommended by the American Medical Association 8
• Laboratories must engage in appropriate quality control procedures to ensure accurate and reproducible results, according to the American Medical Association 8

Clinical Considerations

• Heterophilic antibodies against mouse proteins can cause falsely elevated TSH in some assays, as reported by the American Medical Association 8
• Recent levothyroxine dose adjustments require 6-8 weeks to reach steady state before accurate interpretation, as stated by the American Medical Association 8
• Recovery from severe illness or destructive thyroiditis can cause transient TSH elevation, according to the American Medical Association 8

Reference Range and Interpretation of Serum TSH in Adults

Standard Adult Reference Range

• The American Association of Clinical Endocrinologists endorses a reference range of 0.4–4.5 mIU/L for serum TSH in disease‑free adults, derived from the 2.5th–97.5th percentile of NHANES III data. 9
• In healthy adults the geometric mean TSH is ≈ 1.4 mIU/L, reflecting the central tendency of normal thyroid function. 9
• The lower limit of 0.4 mIU/L remains stable across all adult ages, whereas the upper limit rises progressively with advancing age. 9, 10

Age‑Related Adjustments to the Upper Limit

• Among adults ≥ 80 years, roughly 12 % have TSH > 4.5 mIU/L without underlying thyroid disease, indicating that the standard interval may over‑diagnose older individuals. 10
• Age‑specific upper limits have been proposed: about 3.75 mIU/L at age 40, increasing to 5.0–7.5 mIU/L by ages 80–90. 10

Biological and Methodologic Variability

• Day‑to‑day TSH variability can reach 50 % of the mean, and serial measurements taken at the same time of day may differ by up to 40 %, making a single TSH value insufficient for definitive diagnosis. 9
• Acute illness, hospitalization, certain medications (e.g., dopamine, glucocorticoids, iodine, octreotide, bexarotene), early pregnancy, adrenal insufficiency, and pituitary disorders can transiently suppress or elevate TSH independent of true thyroid pathology. 9, 10

Clinical Application and Screening Algorithms

• Screening asymptomatic adults younger than 80 years should use the standard 0.4–4.5 mIU/L reference range. 9
• For adults aged 80 years and older, an upper limit of 5.0–7.5 mIU/L is recommended to avoid over‑diagnosis, reflecting the natural age‑related rise in TSH. 10
• Abnormal TSH results should be confirmed with repeat testing after 3–6 weeks, as 30–60 % of mildly abnormal values normalize spontaneously. 9
• When TSH is abnormal, measure free T4 concurrently to differentiate subclinical from overt thyroid dysfunction. 9, 10

Recommendations on Routine Screening

• The U.S. Preventive Services Task Force concluded that evidence is insufficient to show that screening for or treating thyroid dysfunction in asymptomatic adults improves quality of life, cardiovascular outcomes, or mortality; therefore, routine screening using stricter cutoffs is not recommended. 9