Unit
- Location and structure: The thyroid gland is a butterfly-shaped organ located in the anterior neck, below the Adam's apple. It consists of two lobes connected by an isthmus and contains follicles that produce thyroid hormones.
- Hormone synthesis pathway: Iodine is absorbed from the bloodstream and transported into follicular cells. Thyroglobulin, a protein produced in the follicles, is iodinated to form precursors that couple to produce triiodothyronine (T3) and thyroxine (T4).
- Role of the hypothalamus-pituitary-thyroid axis: The hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the pituitary to secrete thyroid-stimulating hormone (TSH). TSH then acts on the thyroid to increase hormone production and release.
- Negative feedback regulation: High levels of T3 and T4 suppress TRH and TSH secretion, maintaining hormone balance. This feedback loop is critical for preventing over- or under-production of thyroid hormones.
- Iodine requirement: Adequate dietary iodine intake (150 mcg daily for adults) is essential for hormone production. Iodine deficiency can lead to goiter and hypothyroidism, while excess iodine can trigger hyperthyroidism in susceptible individuals.
- Storage and secretion: The thyroid stores large amounts of hormone precursors within colloid in the follicles. Upon stimulation, hormones are cleaved from thyroglobulin and released into the bloodstream for transport.
- Transport proteins: Over 99% of T3 and T4 in the blood are bound to thyroxine-binding globulin, transthyretin, and albumin. Free fractions (free T3, free T4) are biologically active and measured in diagnostic tests.
- Peripheral conversion: T4 is a prohormone that is converted to the more active T3 in tissues by deiodinase enzymes. This conversion occurs in the liver, kidney, and other tissues, regulating local thyroid hormone action.
- Effects of thyroid hormones on metabolism: T3 and T4 increase basal metabolic rate by stimulating oxygen consumption and heat production in most tissues. They enhance carbohydrate and fat metabolism, promote protein synthesis in some tissues and catabolism in others, and are crucial for normal growth and development, especially in the nervous system.
- Common thyroid disorders: Hypothyroidism results from insufficient hormone production, often due to autoimmune Hashimoto's thyroiditis. Hyperthyroidism involves excess hormone production, frequently caused by Graves' disease. Both conditions disrupt metabolic balance and require careful management with medication or other interventions.
- Calcitonin function: The thyroid gland also produces calcitonin from parafollicular C cells. Calcitonin lowers blood calcium levels by inhibiting bone resorption and increasing renal excretion of calcium. However, its role in human calcium homeostasis is minor compared to parathyroid hormone and vitamin D.
Scope of Practice
Role: Healthcare Coordinator in Endocrine Clinic
Within scope:
- Educating patients on the basic function of the thyroid and the importance of iodine intake.
- Coordinating blood draws for TSH, free T4, and free T3 assessments.
- Documenting patient history related to thyroid symptoms and risk factors.
- Scheduling follow-up appointments based on provider recommendations.
Outside scope:
- Interpreting thyroid function test results or diagnosing disorders.
- Recommending specific dietary changes beyond general healthy eating guidelines.
- Adjusting medication dosages or hormone replacement therapy.
- Performing physical examination of the thyroid gland.
Quick Revision
- The thyroid gland produces T3 and T4 via iodine incorporation into thyroglobulin.
- The HPT axis and negative feedback loops regulate hormone secretion.
- Iodine is essential; deficiency or excess both disrupt thyroid function.
- Transport proteins carry most thyroid hormones; free fractions are active.
- Peripheral conversion of T4 to T3 controls hormone activity at tissue level.