How can I differentiate between primary and secondary thyroid dysfunction based on TFT results?

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) ¹.

• Primary Thyroid Dysfunction: This indicates a problem originating in the thyroid gland itself ¹.
  • Primary Hypothyroidism: Test results typically show a high TSH level, accompanied by a low free thyroxine (FT4) level ¹. This pattern suggests the pituitary gland is working harder to stimulate an underactive thyroid ¹.
  • Primary Hyperthyroidism (Thyrotoxicosis): This is characterised by a low TSH level, with high free thyroxine (FT4) and/or free tri-iodothyronine (FT3) levels ¹. This indicates an overactive thyroid gland suppressing TSH production from the pituitary ¹.
• Secondary Thyroid Dysfunction (Pituitary Disease): This indicates a problem originating in the pituitary gland, which affects TSH production and subsequently thyroid hormone levels ¹. When secondary thyroid dysfunction is suspected, both TSH and FT4 should be measured initially ¹.
  • Secondary Hypothyroidism: Test results would show a low or inappropriately normal TSH level, accompanied by a low free thyroxine (FT4) level ¹. This occurs because the pituitary gland is not producing enough TSH to stimulate the thyroid ¹.
  • 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 ¹. This pattern suggests the pituitary is overproducing TSH, leading to thyroid overactivity ¹.

It is important to note that thyroid tests should be performed on broad indications when thyroid dysfunction is suspected (Hallengren, 1998; Bryer-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 ¹.