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Pitfalls in the measurement and interpretation of thyroid function tests

Thyroid function tests (TFTs) are amongst the most commonly requested laboratory investigations in both primary and secondary care. Fortunately, most TFTs are straightforward to interpret and confirm the clinical impression of euthyroidism, hypothyroidism or hyperthyroidism. However, in an important subgroup of patients the results of TFTs can seem confusing, either by virtue of being discordant with the clinical picture or because they appear incongruent with each other [e.g. raised thyroid hormones (TH), but with non-suppressed thyrotropin (TSH); raised TSH, but with normal TH].

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 Laboratory Testing in Thyroid Conditions - Pitfalls and Clinical Utility

Thyroid disorders are common, affecting more than 10% of people in the US, and laboratory tests are integral in the management of these conditions. The repertoire of thyroid tests includes blood tests for thyroid-stimulating hormone (TSH), free thyroxine, free triiodothyronine, thyroglobulin (Tg), thyroglobulin antibodies (Tg-Ab), thyroid peroxidase antibodies (TPO-Ab), TSH receptor antibodies (TRAb), and calcitonin.

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The Subtle Signs of a Thyroid Disorder

Yet in a significant number of people with thyroid deficiencies, routine blood tests fail to detect insufficient thyroid hormone, leaving patients without an accurate explanation for their symptoms.

How reliable are free thyroid and total T3 hormone assays?

The two major thyroid hormones produced by the thyroid gland are 3,5,3′-triiodothyronine (T3) and thyroxine (T4). The thyroid gland exclusively synthesizes T4, and produces approximately 20% of T3. The majority of T3 is generated by 5′-deiodination catalyzed by type I and type II T4–5′-deiodinases in peripheral tissue.

How to interpret thyroid function tests

Here, we highlight the main causes of commonly encountered patterns of abnormal thyroid function, including so-called ‘anomalous/discordant TFTs’, and propose a simple strategy for their investigation.

Initial Evaluation of Thyroid Function

Thyroid function tests are used in the initial evaluation of thyroid disease. The recommended first test is the measurement of thyroid stimulating hormone (TSH, or thyrotropin), which is generally followed by a thyroxine (T4) test. In limited cases, triiodothyronine (T3) testing may be useful.

Limitations of Thyroid Testing

Most conventional practitioners test TSH (thyroid stimulating hormone) levels as a first port of call when assessing thyroid function, however these recent reviews have shown it to be an unreliable marker of hypothyroidism that may be missing many cases.

Thyroid function testing in primary care: overused and under-evidenced?

Diagnostic testing is increasing in primary care, including for thyroid disease. This study examined which clinical features were associated with an abnormal thyroid stimulating hormone (TSH) result.

Thyroid test and its significance (t3 & t4)

Thyroid hormones are primarily found in a bound form, and a tiny portion, 0.03% of T4 and 0.3% of T3 travel freely (FT3 & FT4) in the blood and have hormonal activity. Moreover, T3 is produced by conversion from T4 in the body.

Your Thyroid May Be Talking, but Is the Doctor Listening?

Thyroid disorders, experts say, are among the most undiagnosed and misdiagnosed problems in the United States. Doctors generally don't examine the thyroid. Disorders come on slowly, and the symptoms can be ambiguous.

Resources

Thyroid.org

Blood tests to measure these hormones are readily available and widely used, but not all are useful in all situations.

NPS MedicineWise

Thyroid disorders can be difficult to detect clinically, but thyroid function tests can assist in making a diagnosis. Measuring thyroid stimulating hormone is the first step. If it is abnormal, free thyroxine should be measured.