GLOSSARY

Hypothyroidism
Hypothyroidism is inadequate thyroid hormone action arising from primary gland failure or central regulatory disease. Clinical manifestations vary and can overlap with many nonthyroid conditions.

From synthesis to tissue action

Thyroid function involves iodide handling, hormone formation in thyroglobulin, secretion, protein binding and local conversion. Transporters, deiodinases and receptor isoforms shape tissue exposure. These steps explain why circulating total hormone and intracellular thyroid action are different measurements.

Specific considerations

Overt primary hypothyroidism usually combines low FT4 with elevated TSH. Central hypothyroidism may show an insufficient TSH response, making the full axis pattern more informative than a TSH result alone.

Interpreting the axis

TSH and free thyroid hormones are interpreted together with symptoms and assay conditions. Illness, medicines and energy deficiency can alter the pattern. A change during heavy training should not automatically be treated as primary thyroid disease.