Biochemistry Unit 13 Thyroid Function Test.pdf

THYROID FUNCTION TEST BBM1233 MEDICAL BIOCHEMISTRY 2  Learning Objectives At the end of this lecture, the student will be able to: Describe the conditions which lead to abnormal thyroid hormone production Interpret the thyroid function tests  HYPERTHYROIDISM 1. Pri...

THYROID FUNCTION TEST BBM1233 MEDICAL BIOCHEMISTRY 2  Learning Objectives At the end of this lecture, the student will be able to: Describe the conditions which lead to abnormal thyroid hormone production Interpret the thyroid function tests  HYPERTHYROIDISM 1. Primary hyperthyroidism Overactive of the thyroid gland leading to excessive synthesis of thyroid hormones (thyrotoxicosis) TSH level: low; free T4 level: high Commonly caused by: Graves’ disease Toxic nodular goiter Functioning thyroid adenoma (benign tumor of the thyroid gland) Graves’ disease  What : autoimmune disease → presence of thyroid-stimulating antibodies (TSA)  How : TSA binds to TSH receptors in the thyroid → stimulate production of thyroid hormone (same as TSH)  Symptoms: exophthalmos (bulging eyes), goitre (thyroid enlargement), rapid pulse, weight loss, excessive sweating, heat intolerance Exophthalmos Exophthalmos Goitre Goitre Toxic nodular goiter  Also known as Plummer’s disease or toxic multinodular goiter  It occurs most often in older adults (over 60 years old).  Due to enlargement of the gland. The gland contains areas that have increased in size and formed nodules  Prevalence increased with iodine insufficiency.  HYPERTHYROIDISM 2. Secondary hyperthyroidism TSH level: high; free T4 level: high Due to diseases of pituitary or hypothalamus TSH secreting pituitary adenoma Thyroid hormone resistance syndrome Chorionic gonadotropin secreting tumors Gestational thyrotoxicosis  HYPOTHYROIDISM 1. Primary hypothyroidism Underactive thyroid gland that produces insufficient amounts of thyroid hormones TSH level: high; free T4 level: low Commonly caused by: Autoimmune mediated destruction of thyroid tissue (Hashimoto’s thyroiditis) Thyroidectomy & radiation therapy Iodine deficiency in diet  Congenital hypothyroidism Babies with defects in the synthesis of thyroid hormone, absent of thyroid gland, TSH receptor mutation If no treatment with replacement hormone within first few weeks of life, leads to severe impaired growth and permanent mental retardation (cretinism)  HYPOTHYROIDISM 2. Secondary hypothyroidism TSH level: low; free T4 level: low Due to diseases of pituitary or hypothalamus Hypopituitarism (caused by tumors, pituitary surgery or irradiation) Hypothalamic diseases (caused by tumors or trauma)  THYROID FUNCTION TESTS Group of tests done to evaluate the functional capacity of thyroid gland Thyroid function tests are used to: Detect and diagnose thyroid disease Monitor response to treatment of thyroid disorders Evaluate the prognosis of thyroid cancer  THYROID FUNCTION TESTS Two tests used as a standard profiling of thyroid function: Thyroid stimulating hormone (TSH) Free thyroxine (FT4) Total T4 (tT4) & total T3 (tT3) has major disadvantage: E.g. a slightly elevated [tT4] (mild hyperthyroidism) can occur with normal thyroid function if there is an ↑ in [plasma binding protein]  Thyroxine-binding globulin (TBG) level may ↑ by factors such as: Pregnancy Oral contraceptive pills containing estrogen May produce FALSE POSITIVE DIAGNOSIS of hyperthyroidism TBG level may ↓ by factors such as: Protein-losing enteropathy Malnutrition Malabsorption ELISA