Thyroid Function Tests

• Thyroid function tests can assess abnormal thyroid function and identify its cause.

Thyroid Axis Physiology

• The hypothalamus releases thyrotropin-releasing hormone (TRH).
• TRH stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH).
• TSH stimulates the thyroid gland to release triiodothyronine (T3) and thyroxine (T4).
• The hypothalamus and anterior pituitary respond to T3 and T4 by suppressing the release of TRH and TSH, lowering T3 and T4 levels.
• Lower T3 and T4 levels offer less suppression of TRH and TSH, causing more of these hormones to be released, resulting in a rise of T3 and T4.
• This negative feedback loop ensures thyroid hormone levels remain within normal limits.
• Negative feedback occurs when the end hormones (T3 and T4) suppress the release of the controlling hormones (TRH and TSH).

Hormone Tests

• Thyroid-stimulating hormone (TSH) is a screening test for thyroid disease.
• Abnormal TSH levels prompt measurement of triiodothyronine (T3) and thyroxine (T4) for further information.

Primary Hyperthyroidism

• The thyroid behaves abnormally, producing excessive thyroid hormones.
• High T3 and T4 suppress TSH, leading to a low TSH level.
• "GIST" mnemonic for top causes:
  • Graves’ disease
  • Inflammation (thyroiditis)
  • Solitary toxic thyroid nodule
  • Toxic multinodular goitre

Secondary Hyperthyroidism

• The pituitary behaves abnormally, producing excessive TSH (e.g., pituitary adenoma).
• This stimulates the thyroid gland to produce excessive thyroid hormones.
• TSH, T3, and T4 levels are all elevated.

Primary Hypothyroidism

• The thyroid behaves abnormally, producing inadequate thyroid hormones.
• Absent negative feedback results in increased TSH production.
• TSH is raised, while T3 and T4 are low.
• Top causes include:
  • Hashimoto’s thyroiditis
  • Iodine deficiency
  • Treatments for hyperthyroidism

Secondary Hypothyroidism

• The pituitary behaves abnormally, producing inadequate TSH (e.g., after surgical removal of the pituitary).
• This leads to under-stimulation of the thyroid gland, resulting in insufficient thyroid hormones.
• TSH, T3, and T4 levels are all low.

TSH, T3 and T4 Levels in Thyroid Conditions

• Primary Hyperthyroidism: Low TSH, high T3 and T4
• Secondary Hyperthyroidism: High TSH, high T3 and T4
• Primary Hypothyroidism: High TSH, low T3 and T4
• Secondary Hypothyroidism: Low TSH, low T3 and T4

Antibodies

• Anti-thyroid peroxidase (anti-TPO) antibodies target the thyroid gland and are relevant in autoimmune thyroid disease.
• They are commonly present in Graves’ disease and Hashimoto’s thyroiditis.
• Anti-thyroglobulin (anti-Tg) antibodies target thyroglobulin, a protein in the thyroid gland.
• They can be present in normal individuals but are usually raised in Graves’ disease, Hashimoto’s thyroiditis, and thyroid cancer.
• TSH receptor antibodies mimic TSH, bind to the TSH receptor, and stimulate thyroid hormone release.
• These antibodies cause Grave’s disease and are present in this condition.

Imaging

• Ultrasound of the thyroid gland helps diagnose thyroid nodules and differentiate between cystic and solid nodules.
• It can also guide a biopsy of a thyroid lesion.
• Radioisotope scans investigate hyperthyroidism and thyroid cancers.
• Radioactive iodine is administered orally or intravenously and taken up by thyroid cells.
• The more active the thyroid cells, the faster they take up radioactive iodine.
• A gamma camera detects gamma rays emitted from the radioactive iodine to give insight into the thyroid gland.
• Diffuse high uptake is found in Grave’s Disease.
• Focal high uptake is found in toxic multinodular goitre and adenomas.
• "Cold" areas (abnormally low uptake) can indicate thyroid cancer.