Back to the Basics: Thyroid Gland Structure, Function and Pathology

1. Janelle M. Chiasera, PhD, MT (ASCP)[⇑][1] 1. University of Alabama at Birmingham, Birmingham, Alabama 1. Address for Correspondence: Janelle M. Chiasera, PhD, MT(ASCP) , Chair and Professor, University of Alabama at Birmingham, 1705 University Blvd, SHPB 431, Birmingham, Alabama 35294, 205-975-3111, chiasera{at}uab.edu 1. Explain the HPT feedback system involving the thyroid gland. Include the hormone produced from each gland and the effect of that hormone on the other unit. 2. Describe thyroid hormone synthesis including a description of the 5 steps of hormone production and the rate limiting step to the production of thyroid hormones. 3. Differentiate between hyperthyroidism and hypothyroidism including associated laboratory data, signs and symptoms presented, primary cause, and treatment. 4. Explain the importance of neonatal screening and indicate the type of screening method used by the U.S. 5. Explain the difference between overt and subclinical thyroid disorders and euthyroid sick syndrome and explain what the laboratory data would look like in both conditions. The thyroid gland is one of many glands associated with the endocrine system and it is responsible for the production and secretion of the thyroid hormones, triiodothyronine (T3) and thyroxine (T4). The thyroid gland is located at the front of the neck and is bilobular in structure such that it has a butterfly appearance. The thyroid gland is made up of two cell types, follicular and parafollicular cells. The follicular cells are responsible for producing thyroid hormones. The follicular cells enclose a space called the colloid which contains stored thyroglobulin, a glycoprotein that contains the precursors T3 and T4. The parafollicular cells, also known as C-cells, secrete the hormone calcitonin, a hormone responsible for the regulation of calcium.1-3 The thyroid gland, like other glands of the endocrine system, is controlled through a feedback system involving the hypothalamus, the pituitary, and the target gland (the thyroid). The relationship between the hypothalamus, the pituitary and the thyroid gland is referred to as the HPT axis. The hypothalamus is responsible for producing thyrotropin-releasing hormone (TRH), a tripeptide which is secreted into the venous system that drains to the pituitary gland. At the pituitary, TRH binds to receptors in thyrotroph cells causing the production and secretion of thyroid stimulating hormone (TSH), also known as thyrotropin. Thyroid stimulating hormone binds to TSH receptors in the follicular cells of the thyroid gland causing the production and secretion of thyroid hormones, T3 and T4. As with all endocrine glands, the thyroid gland exhibits both negative and positive… ABBREVIATIONS : AAP - American Academy of Pediatrics; DIT - diiodotryrosine; fT3 - Free T3; fT4 - Free T4; HPT - hypothalamus, pituitary, thyroid axis; MIT - monoiodotryrosine; rT3 - reverse T3; ESS - euthyroid sick syndrome; T3 - triiodothyronine; T4 - thyroxine; TBG - thyroxine-binding globulin; TBPA - thyroxine-binding prealbumin; THBR - thyroid hormone binding ratio; TRH - thyrotropin-releasing hormone; TSH - thyroid stimulating hormone; TT3 - total T3; TT4 - total T4 1. Explain the HPT feedback system involving the thyroid gland. Include the hormone produced from each gland and the effect of that hormone on the other unit. 2. Describe thyroid hormone synthesis including a description of the 5 steps of hormone production and the rate limiting step to the production of thyroid hormones. 3. Differentiate between hyperthyroidism and hypothyroidism including associated laboratory data, signs and symptoms presented, primary cause, and treatment. 4. Explain the importance of neonatal screening and indicate the type of screening method used by the U.S. 5. Explain the difference between overt and subclinical thyroid disorders and euthyroid sick syndrome and explain what the laboratory data would look like in both conditions. [1]: #corresp-1