Thyroid function tests

Thyroid function tests are blood tests used to assess thyroid activity, chiefly by measuring thyroid-stimulating hormone and circulating thyroid hormones.

Thyroid-stimulating hormone: A pituitary hormone that stimulates the thyroid gland to produce and release thyroid hormones. TSH is usually the first-line measurement because its level responds sensitively to changes in thyroid hormone.
Hypothyroidism: A condition in which the body has insufficient thyroid hormone activity. High TSH with low free T4 commonly indicates primary hypothyroidism.
Thyroid-binding globulin: The main plasma protein that carries thyroxine and triiodothyronine in the bloodstream. Changes in this protein can alter total hormone measurements without changing free hormone levels.
Thyroiditis: Inflammation of the thyroid gland that can cause temporary release or reduced production of thyroid hormones. Its evolving phases can make test results change from apparent hyperthyroidism to hypothyroidism.
Free thyroxine: The unbound fraction of thyroxine circulating in blood and available to tissues. Free T4 helps distinguish thyroid hormone deficiency from pituitary or hypothalamic causes of abnormal TSH.
Hyperthyroidism: A condition in which the thyroid gland produces excessive thyroid hormones. Low TSH with elevated thyroid hormones is a typical biochemical pattern.
Thyroid peroxidase antibodies: Autoantibodies directed against thyroid peroxidase, an enzyme involved in thyroid hormone synthesis. Their presence supports autoimmune thyroiditis when thyroid function tests are abnormal.
Non-thyroidal illness syndrome: Changes in thyroid hormone measurements during severe illness without primary thyroid gland dysfunction. It can produce abnormal results that do not represent a thyroid disorder.
Triiodothyronine: A thyroid hormone more biologically active than thyroxine, circulating in free and protein-bound forms. T3 measurement can uncover or clarify hyperthyroidism when TSH is suppressed.
Subclinical hypothyroidism: An elevated TSH level with thyroid hormone concentrations still within their reference ranges. This borderline pattern makes treatment decisions depend on factors beyond hormone levels alone.

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