What are the key diagnostic criteria for confirming a diagnosis of hypothyroidism in adults?

Key diagnostic criteria for confirming hypothyroidism in adults primarily involve biochemical testing of thyroid function. The initial and most critical test is the measurement of serum thyroid-stimulating hormone (TSH). An elevated TSH above the reference range indicates hypothyroidism, prompting measurement of free thyroxine (FT4) in the same sample to differentiate between overt and subclinical hypothyroidism. Overt hypothyroidism is confirmed when TSH is elevated and FT4 is below the reference range, while subclinical hypothyroidism is characterized by elevated TSH with normal FT4 levels ¹,².

In adults where secondary hypothyroidism (pituitary or hypothalamic disease) is suspected, both TSH and FT4 should be measured simultaneously, as TSH may be low or inappropriately normal despite low FT4 ²,¹.

Additional diagnostic considerations include the presence of thyroid peroxidase antibodies (TPOAbs), which support an autoimmune etiology such as Hashimoto’s thyroiditis when positive in adults with elevated TSH ¹,². However, TPOAbs testing is not repeated routinely once positive.

Clinical context is important: symptoms suggestive of hypothyroidism (fatigue, weight gain, cold intolerance, constipation, dry skin) alongside biochemical abnormalities strengthen the diagnosis, but symptoms alone are insufficient for diagnosis ¹,².

Repeat testing is recommended to confirm persistent TSH elevation, typically on two occasions at least 3 months apart, to exclude transient thyroid dysfunction ¹,².

In summary, the key diagnostic criteria for hypothyroidism in adults are:

• Elevated serum TSH above the reference range
• Low serum FT4 for overt hypothyroidism; normal FT4 for subclinical hypothyroidism
• Positive TPOAbs supporting autoimmune thyroiditis
• Confirmation on repeat testing after 3 months
• Consideration of clinical symptoms and exclusion of secondary causes

This approach aligns with NICE guidelines and British Thyroid Association consensus, supported by recent clinical practice guidelines emphasizing the importance of TSH and FT4 measurement and antibody testing in diagnosis (Bekkering et al., 2019) ¹,²(Bekkering et al., 2019).