Interpreting thyroid-stimulating hormone (TSH), free thyroxine (Free T4), and free triiodothyronine (Free T3) levels in patients with suspected hypothyroidism involves a hierarchical approach, with TSH typically serving as the initial screening test NICE NG145.
- Thyroid-Stimulating Hormone (TSH): TSH is the primary screening test for adults when secondary thyroid dysfunction (pituitary disease) is not suspected NICE NG145. A TSH level above the reference range is the key indicator of primary hypothyroidism NICE NG145. Conversely, a TSH level below the reference range, especially when accompanied by a low Free T4, suggests secondary (central) hypothyroidism, which involves pituitary disease NICE NG145 Trainer & Howard 1983. In subclinical hypothyroidism, TSH is elevated, but Free T4 levels remain within the reference range NICE NG145.
- Free Thyroxine (Free T4): Free T4 is measured to confirm the diagnosis of primary hypothyroidism when TSH is found to be above the reference range NICE NG145. A low Free T4 in conjunction with an elevated TSH confirms overt primary hypothyroidism NICE NG145 Perry 1994. For suspected secondary hypothyroidism, both TSH and Free T4 are measured initially, with a low Free T4 alongside a low or inappropriately normal TSH being indicative NICE NG145 Trainer & Howard 1983. In cases of subclinical hypothyroidism, Free T4 levels are typically within the normal reference range despite an elevated TSH NICE NG145.
- Free Triiodothyronine (Free T3): Free T3 generally plays a less significant role in the initial diagnosis of hypothyroidism compared to TSH and Free T4 Trainer & Howard 1983. It is often the last thyroid hormone to fall in developing hypothyroidism Perry 1994. NICE guidelines recommend measuring Free T3 in the same sample if TSH is below the reference range, which is primarily relevant for suspected hyperthyroidism or when investigating a suppressed TSH NICE NG145. For suspected hypothyroidism, its measurement is not typically a primary diagnostic step unless TSH is low and secondary dysfunction is suspected NICE NG145.
It is important to note that thyroid function tests should generally not be performed during an acute illness unless the illness is suspected to be due to thyroid dysfunction, as acute illness can affect results NICE NG145. Additionally, patients should be asked about biotin intake, as high consumption from dietary supplements can lead to falsely high or low test results NICE NG145. If symptoms worsen or new symptoms develop, tests may be repeated, but no sooner than 6 weeks from the most recent test NICE NG145.