Journal List > J Korean Med Assoc > v.61(4) > 1043282

Yoon and Kang: Interpretation of puzzling thyroid function tests

Abstract

With the generalized use of highly sensitive thyroid stimulating hormone (TSH) and free thyroid hormone assays, most thyroid function tests (TFTs) are straightforward to interpret and confirm the clinical impressions of thyroid diseases. However, in some patients, TFT results can be perplexing because the clinical picture is not compatible with the tests or because TSH and free T4 are discordant with each other. Optimizing the interpretation of TFTs requires a complete knowledge of thyroid hormone homeostasis, an understanding of the range of tests available to the clinician, and the ability to interpret biochemical abnormalities in the context of the patient's clinical thyroid status. The common etiologic factors causing puzzling TFT results include intercurrent illness (sick euthyroid syndrome), drugs, alteration in normal physiology (pregnancy), hypothalamic-pituitary diseases, rare genetic disorders, and assay interference. Sick euthyroid syndrome is the most common cause of TFT abnormalities encountered in the hospital. In hypothalamic-pituitary diseases, TSH levels are unreliable. Therefore, it is not uncommon to see marginally high TSH levels in central hypothyroidism. Drugs may be the culprit of TFT abnormalities through various mechanisms. Patients with inappropriate TSH levels need a differential diagnosis between TSH-secreting pituitary adenoma and resistance to thyroid hormone. Sellar magnetic resonance imaging, serum α-subunit levels, serum sex hormone-binding globulin levels, a thyrotropin-releasing hormone stimulation test, trial of somatostatin analogues, and TR-β sequencing are helpful for the diagnosis, but it may be challenging. TFTs should be interpreted based on the clinical context of the patient, not just the numbers and reference ranges of the tests, to avoid various pitfalls of TFTs and unnecessary costly evaluations and therapies.

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Table 1.
Basic interpretation of thyroid function tests
TSH Free T4 Total T3 Clinical diseases
Euthyroid status
Thyrotoxicosis (Graves’ disease is the most common cause)
  Subclinical hyperthyroidism
      Sick euthyroid syndrome
↓(or ↔)a) ↓ (or ↔) Central hypothyroidism
      Sick euthyroid syndrome
↓ or ↔ Primary hypothyroidism
  Subclinical hypothyroidism
↑ or ↔ TSH-secreting pituitary adenoma
      Resistance to thyroid hormone

TSH, thyroid stimulating hormone.

a) TSH may be slightly increased in central hypothyroidism.

Table 2.
Drugs affecting thyroid function
Mechanism Drugs
Decrease TSH secretion Dopamine, glucocorticoid, octreotide
Thyroid hormone secretion Decrease: Iodide, lithium, amiodarone, aminoglutethimide
  Increase: Iodide, amiodarone
Inhibition of T4 to T3 conversion Propylthiouracil, glucocorticoids, amiodarone, propranolol
Increased hepatic metabolism of thyroid hormones Phenobarbital, rifampin, phenytoin, carbamazepine
Thyroid hormone transport Increased TBG: estrogen, tamoxifen, raloxifene, methadone, mitotane, fluorouracil
  Decreased TBG: androgens, anabolic steroids, nicotinic acid, glucocorticoids
  Displacement from binding sites: furosemide, heparin, mefenamic acid, salicylates
Decreased T4 absorption from gastrointestinal tract Aluminium hydroxide, ferrous sulfate, sucralfate, cholestyramine, proton pump inhibitors, cation exchange resin
Induce thyroid autoimmunity Interferon-alfa, interleukin-2, immune check point inhibitors
Destructive toxic effects (various mechanisms) Tyrosine kinase inhibitors (sunitinib, sorafenib, pazofanib, axitinib, vandetanib, cabozantinib, imatinib)

TSH, thyroid stimulating hormone; TBG, thyroxine-binding globulin.

Table 3.
Serial thyroid function tests results of the case 1
  4 Months ago 1 Month ago On presentation
Free T4 (0.9-1.7 ng/dL) 0.028 0.49 1.57
TSH (0.4-4.5 μU/mL) 12.9 6.37 1.08

TSH, thyroid stimulating hormone.

Table 4.
Thyroid function test results of the case 2
Test Results
Free T4 (0.9-1.7 ng/dL) 0.61
Total T3 (60-160 ng/dL) 220
TSH (0.4-4.5 μU/mL) 1.2

TSH, thyroid stimulating hormone.

Table 5.
Thyroid function test results of the case 3
Test Results
Free T4 (0.9-1.7 ng/dL) 4.6
Total T3 (60-160 ng/dL) 207
TSH (0.4-4.5 μU/mL) 6.68

TSH, thyroid stimulating hormone.

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