Diagnosis of hypothyroidism in nonpregnant adults

LLN: lower limit of normal reference range; T3: triiodothyronine; T4: thyroxine; TSH: thyroid-stimulating hormone; ULN: upper limit of normal reference range.

* In general, TSH should not be assessed in seriously ill patients unless there is a strong suspicion of thyroid dysfunction, since there are many other factors in acutely or chronically ill euthyroid patients that influence TSH secretion. Some hospitalized patients have transient elevations in serum TSH concentrations (up to 20 mU/L) during recovery from severe nonthyroidal illness. Refer to UpToDate content on nonthyroidal illness.

¶ The serum TSH distribution shifts toward higher values with age, independent of the presence of antithyroid antibodies. The ULN could be as high as 6 to 8 mU/L in healthy octogenarians.

Δ If TSH result is discordant with initial testing, repeat again and consider causes of evolving thyroid function such as a destructive thyroiditis or nonthyroidal illness.

◊ A patient with a low free T4 and a slightly high serum TSH concentration (eg, 5 to 10 mU/L [normal range 0.5 to 5 mU/L]) could have either primary or central hypothyroidism, although central hypothyroidism is rare. Central hypothyroidism should be suspected in patients with known hypothalamic or pituitary disease and when there are symptoms and signs of other hormonal deficiencies.

§ Differential diagnosis includes recovery from nonthyroidal illness, recovery from the hyperthyroid phase of thyroiditis, or rarely, central hypothyroidism (TSH may be mildly elevated with free T4 in the LLN) or resistance to TSH.

¥ Assay interference due to heterophilic antibodies or antibodies to TSH.