Thyroid and Parathyroid Hormone

Flashcards based on lecture notes about Thyroid and Parathyroid Hormone.

Thyroid Gland

Gland consisting of two lobes located in the lower part of the neck, just below the voice box (larynx).

Isthmus

Narrow band that connects the two lobes of the thyroid gland.

Cretinism

Condition due to lack of thyroxine from birth or before, leading to severe mental defects and stunted growth.

Follicle

The fundamental structural unit of the thyroid gland.

Follicular cells

Secrete T3 and T4 in the thyroid gland.

Parafollicular cells

Secrete Calcitonin in the thyroid gland.

Thyroglobulin

Acts as a preformed matrix containing tyrosyl groups and glycoproteins, stored in the follicular colloid of the thyroid gland.

Iodine

Most important element in the biosynthesis of thyroid hormones; found in seafood, dairy products, and iodine-enriched foods.

Monoiodotyrosine(MIT) and Diiodotyrosine (DIT)

Formation resulting from iodination of tyrosine residues in thyroglobulin.

Type 1 iodothyronine 5-deiodinase

Most abundant form of iodothyronine 5-deiodinase, found mostly in the liver and responsible for the conversion of T4 to the circulating T3 pool.

Type 2 iodothyronine 5-deiodinase

Iodothyronine 5-deiodinase found in the brain and pituitary that maintains constant levels of T3 in the brain.

Thyroxine-Binding Globulin (TBG)

Transports the majority of T4, also transports T3 though its affinity is less than T4.

Thyroxine-Binding Prealbumin

Also known as transthyretin; transports 15 to 20% of total T4.

Thyroxine-Binding Albumin

Transports the remaining T3 and small portion of T4.

Free Hormones (FT3, FT4)

Physiologically active portions of thyroid hormones.

Protein bound hormones

Metabolically inactive portions of the thyroid hormones.

Hypothalamic-pituitary-thyroid axis

Neuroendocrine system that regulates the production and secretion of the thyroid hormones.

Triiodothyronine (T3)

Hormone with the greatest thyroid hormonal activity.

Tetraiodothyronine (T4)

Principal secretory product of the thyroid gland and a prohormone for T3 production.

TSH Receptor (TSHR)

Autoantigen responsible for autoimmune thyroid disorders

Hyperthyroidism and Hypothyroidism

Major thyroid disorders

Hyperthyroidism

Condition characterized by excessive circulating thyroid hormones.

Hypothyroidism

Condition characterized by deficient circulating thyroid hormones.

Primary Hyperthyroidism

Elevated FT4, Decreased TSH, Decreased TRH

Secondary Hyperthyroidism

Increased FT4, Increased TSH,(due to primary lesion in the pituitary gland)

Thyrotoxicosis

Applied to a group of syndromes caused by high levels of free thyroid hormones in the circulation. TSH is decreased, FT4 is increased, and FT3 may be normal or increased.

Graves’ Disease (Diffuse Toxic Goiter)

Most common cause of thyrotoxicosis; an autoimmune disease.

Exophthalmos

Bulging of the eyes, a feature of Graves' Disease.

Riedel’s Thyroiditis

Thyroid turns into stony hard consistency.

Subacute granulomatous thyroiditis

Associated with neck pain, low-grade fever, and swings in thyroid function tests; antithyroid antibodies are absent, and ESR and thyroglobulin levels are elevated.

Hypothyroidism

Condition that develops whenever inadequate amounts of thyroid hormone are available to tissues.

Primary Hypothyroidism

Due to lack of elemental iodine, decreased T3 and T4, and increased TSH.

Hashimoto’s Disease

Autoimmune thyroiditis where the thyroid is replaced by lymphoid tissue.

Myxedema

Describes the nonpitting swelling of the skin in hypothyroidism; skin becomes infiltrated by mucopolysaccharides.

Secondary Hypothyroidism

Due to pituitary destruction or pituitary adenoma; T3 and T4 are decreased, and TSH is decreased.

Tertiary Hypothyroidism

Due to hypothalamic disease; T3 and T4 are decreased, and TSH is decreased.

Congenital Hypothyroidism / Cretinism

Defects in the development or function of the thyroid gland leading to retarded physical and mental development.

Subclinical Hypothyroidism

Condition with T3 and T4 within normal ranges but TSH is slightly elevated.

TSH Test

Most important thyroid function test; highly sensitive for the detection of primary thyroid disorders.

Radioactive Iodine Uptake (RAIU)

Measures the ability of the thyroid gland to trap and organify iodine; helpful in establishing the cause of thyrotoxicosis.

Thyroglobulin (Tg) assay

Normally used as a tumor marker of thyroid cancer and in monitoring the course of treatment.

Reverse T3 (rT3)

Used to assess borderline or conflicting laboratory results; identifies patients with nonthyroidal illness.

Free Thyroxine Index (FT4I)

Also known as T7, assess the level of free T4 in the blood and important in correcting euthyroid individuals.

T3 Uptake Test

Measures the number of available binding sites of the thyroxine-binding proteins, particularly TBG.

Fine Needle Aspiration

Most accurate tool in the evaluation of thyroid nodules.

Thyroxine Binding Globulin (TBG) test

Useful in distinguishing between hyperthyroidism causing high thyroxine levels and euthyroidism with increased binding by TBG and increased T4.

Parathyroid Gland

Secretes Parathyroid Hormone (PTH)

Role of Parathyroid Hormone

Prime role is to prevent hypocalcemia; promotes bone resorption and increases renal reabsorption of calcium.

Primary Hyperparathyroidism

Physiologic defect lies within the parathyroid glands themselves; most common cause of hypercalcemia.

Secondary Hyperparathyroidism

Develops in response to decreased serum calcium; common with chronic renal failure and vitamin D deficiency.

Tertiary Hyperparathyroidism

Hypoparathyroidism that occurs with secondary hyperparathyroidism.

Hypoparathyroidism

Condition due to accidental damage to the parathyroid glands during surgery or autoimmune parathyroid destruction.