What hormones does the thyroid gland regulate?

Thyroid Hormones and Their Regulation

The thyroid gland primarily produces two essential hormones: thyroxine (T4) and triiodothyronine (T3), which are critical for regulating metabolic processes throughout the body. The thyroid gland primarily produces thyroxine (T4) and triiodothyronine (T3), with T4 constituting about 80% and T3 about 20% of thyroid hormone production. 1

Main Thyroid Hormones

• Thyroxine (T4) - The major hormone secreted by the thyroid gland, accounting for approximately 80% of thyroid hormone production 1
• Triiodothyronine (T3) - The biologically active form that accounts for about 20% of direct thyroid production, with the majority (about 80%) being produced outside the thyroid through deiodination of T4 1, 2

Synthesis and Structure of Thyroid Hormones

• Thyroid hormones are iodinated derivatives of the amino acid tyrosine 3
• The synthesis process involves several key steps:
  • Active iodide transport into the thyroid gland 4
  • Iodide oxidation by thyroid peroxidase (TPO) enzyme 4
  • Iodination of tyrosyl residues in thyroglobulin (Tg) to produce monoiodotyrosine (MIT) and diiodotyrosine (DIT) 4, 5
  • Coupling of these iodotyrosines: two DIT molecules form T4, while MIT and DIT coupling forms T3 5

Regulation of Thyroid Hormone Production

• Thyroid hormone production is primarily regulated by thyroid stimulating hormone (TSH) from the pituitary gland 6
• TSH controls multiple aspects of thyroid hormone synthesis:
  • Stimulates iodide uptake into the thyroid 6
  • Activates thyroid peroxidase (TPO) enzyme required for MIT and DIT production 4
  • Regulates oxidation and organification of iodide 6
  • Controls secretion of T4 and T3 into circulation 6
  • Influences post-translational modifications of thyroglobulin that enhance T3 formation 5

Metabolism and Function of Thyroid Hormones

• T3 is the physiologically active form responsible for most thyroid hormone effects 7
• T3 and T4 regulate metabolic rate by affecting various enzymes involved in fat and carbohydrate metabolism 6
• The liver is the major site for thyroid hormone metabolism, with approximately 80% of circulating T3 derived from peripheral deiodination of T4 7, 2
• Thyroid hormones act by:
  • Diffusing into cell nuclei 7
  • Binding to thyroid receptor proteins attached to DNA 7
  • Activating gene transcription and protein synthesis 7

Clinical Significance

• Thyroid dysfunction can manifest as hypothyroidism or hyperthyroidism 6
• Adequate iodine intake is essential for proper thyroid hormone synthesis, with daily requirements of 150 μg/day in adults 6
• Iron and selenium are also crucial for proper thyroid function:
  • Iron deficiency can impair thyroid metabolism 6
  • Selenium is required for the deiodination of T4 to T3 via the selenoenzyme Type 1 5'-deiodinase 6

Thyroid Hormone Testing

• Free T4 (FT4) and free T3 (FT3) measurements are preferred over total hormone measurements for diagnostic purposes 1
• TSH levels alone are not a sensitive indicator of iodine status, as concentrations are usually maintained within normal range despite frank iodine deficiency 6

Thyroid hormones play vital roles in numerous physiological processes, and their proper regulation is essential for maintaining overall health and metabolic function.