Dr Kola Tytler MBBS CertHE MBA MRCGPClinical Lead • iatroX
Differentiating between primary and secondary thyroid dysfunction based on thyroid function test results primarily involves assessing the levels of Thyroid-Stimulating Hormone (TSH), free thyroxine (FT4), and free tri-iodothyronine (FT3) NICE NG145.
- Primary Thyroid Dysfunction: This indicates a problem originating in the thyroid gland itself NICE NG145.
- Primary Hypothyroidism: Test results typically show a high TSH level, accompanied by a low free thyroxine (FT4) level NICE NG145. This pattern suggests the pituitary gland is working harder to stimulate an underactive thyroid NICE NG145.
- Primary Hyperthyroidism (Thyrotoxicosis): This is characterised by a low TSH level, with high free thyroxine (FT4) and/or free tri-iodothyronine (FT3) levels NICE NG145. This indicates an overactive thyroid gland suppressing TSH production from the pituitary NICE NG145.
- Secondary Thyroid Dysfunction (Pituitary Disease): This indicates a problem originating in the pituitary gland, which affects TSH production and subsequently thyroid hormone levels NICE NG145. When secondary thyroid dysfunction is suspected, both TSH and FT4 should be measured initially NICE NG145.
- Secondary Hypothyroidism: Test results would show a low or inappropriately normal TSH level, accompanied by a low free thyroxine (FT4) level NICE NG145. This occurs because the pituitary gland is not producing enough TSH to stimulate the thyroid NICE NG145.
- Secondary Hyperthyroidism: Although rare, this would present with an inappropriately normal or high TSH level, alongside high free thyroxine (FT4) and/or free tri-iodothyronine (FT3) levels NICE NG145. This pattern suggests the pituitary is overproducing TSH, leading to thyroid overactivity NICE NG145.
It is important to note that thyroid tests should be performed on broad indications when thyroid dysfunction is suspected Hallengren 1998Bryer-Ash 2001. However, testing should generally be avoided during an acute illness unless the illness is suspected to be due to thyroid dysfunction, as acute illness can affect test results NICE NG145.