Initial testing for thyroid disorders

Thyroid disorders can affect almost all body systems, causing diverse and nonspecific presentations, so diagnosis requires laboratory testing for confirmation.

The most useful initial test for a suspected thyroid disorder is serum thyroid stimulating hormone (TSH) concentration.

If the serum TSH concentration is within the reference range, a primary thyroid disorder is unlikely, but a central thyroid disorder cannot be excluded. Additional thyroid function tests are usually unnecessary, unless a central disorder is suspected based on presentation (see Overview of hypopituitarism for signs and symptoms).

If the serum TSH concentration is above or below the reference range, additional thyroid function tests (eg serum free thyroxine [T₄] concentration) are usually necessary to differentiate the diagnosis and to confirm that the abnormal serum TSH concentration is persistent. Interpretation of serum TSH and free T4 concentrations outlines the interpretation of serum T₄ concentration in the context of the serum TSH concentration.

Other tests such as thyroid peroxidase antibody and TSH-receptor antibody are useful to further differentiate the diagnosis. Their use is discussed in the topics Hypothyroidism and Thyrotoxicosis and hyperthyroidism.

Thyroid ultrasound is useful for patients with known or palpable thyroid nodules to assess and define the abnormality. It has no role in the diagnosis of thyroid disorders in the absence of known or palpable thyroid nodules.

Note: Thyroid ultrasound has no role in the diagnosis of thyroid disorders in the absence of known or palpable thyroid nodules.

Table 1. Interpretation of serum TSH and free T₄ concentrations
Printable Table
High serum TSH concentration Normal serum TSH concentration Low serum TSH concentration
High serum TSH concentration
Serum free T₄ concentration	Possible interpretations
high serum free T₄ concentration	possible laboratory error secondary hyperthyroidism (TSH-secreting pituitary adenoma) [NB1] thyroid hormone resistance [NB1] intermittent poor adherence to levothyroxine
normal serum free T₄ concentration	subclinical primary hypothyroidism (also called mild thyroid failure) recent severe nonthyroidal illness
low serum free T₄ concentration	overt primary hypothyroidism recent severe nonthyroidal illness antithyroid overtreatment
Normal serum TSH concentration
Serum free T₄ concentration	Possible interpretations
high serum free T₄ concentration	possible laboratory error secondary hyperthyroidism (TSH-secreting pituitary adenoma) [NB1] thyroid hormone resistance [NB1] sampling within 6 hours of levothyroxine dose
normal serum free T₄ concentration	normal thyroid function
low serum free T₄ concentration	secondary hypothyroidism (pituitary cause) or tertiary hypothyroidism (hypothalamic cause) severe nonthyroidal illness use of drugs that affect serum free T₄ concentration (see Drugs that influence thyroid hormones) antithyroid overtreatment
Low serum TSH concentration
Serum free T ₄ concentration	Possible interpretations
high serum free T₄ concentration	overt primary hyperthyroidism [NB2] [NB3] levothyroxine overtreatment
normal serum free T₄ concentration	subclinical primary hyperthyroidism [NB3] levothyroxine overtreatment nonthyroidal illness treated secondary hypothyroidism (pituitary cause) or tertiary hypothyroidism (hypothalamic cause) recently started antithyroid drug
low serum free T₄ concentration	secondary hypothyroidism (pituitary cause) or tertiary hypothyroidism (hypothalamic cause) severe nonthyroidal illness
Note: T₄ = thyroxine; TSH = thyroid stimulating hormone NB1: Secondary hyperthyroidism and thyroid hormone resistance are rare. Contact the laboratory for confirmation of test results; laboratory error can distort results in up to 5% of tests. NB2: TSH is usually suppressed (serum concentration less than 0.05 milliunits/L, and at least less than 0.1 milliunits/L), rather than just low (serum concentration between the lower limit of the normal range and 0.1 milliunits/L). NB3: Primary hyperthyroidism includes Graves disease, toxic multinodular goitre and toxic adenoma.