Thyroid Hormones and Metabolism

Thyroid Metabolic Hormones

• Thyroid hormones regulate oxygen consumption in most of the body's cells.
• They participate in the metabolism of proteins, lipids, and carbohydrates, essential for normal organ and system function.

Types of Thyroid Hormones

• Thyroid hormones are life regulators.
• They are macromolecules that travel through the blood to all body cells.
• These hormones are essential for cell development and participate in functions like energy and temperature regulation.

Thyroid Hormones and Metabolism

• Thyroid hormones regulate metabolism in animals and humans.
• Metabolism is determined by measuring oxygen used by the body over time.
• The measurement is known as basal metabolism (BM) when done at rest.
• Iodine is needed to produce thyroid hormones.
• The thyroid gland contains most of the body's iodine.

Synthesis of Thyroid Hormones

• Synthesis requires iodine uptake through the basolateral membrane.
• It passes into the thyrocyte, transports through the cell, and exits through the apical membrane into the follicular lumen.

Secretion of Thyroid Hormones

• Secretion is regulated by the pituitary hormone TSH (thyroid-stimulating hormone).
• It is regulated by hypothalamic TRH (thyrotropin-releasing hormone).
• TRH acts on pituitary thyrotrophs and stimulates TSH production.

Thyroxine vs. Triiodothyronine

• Thyroxine (T4) makes up 93% and persists longer, indicating a larger quantity in blood.
• Triiodothyronine (T3) makes up 7% and enables faster action.
• T3 is four times more potent.

Physiological Functions of Thyroid Hormones

• It stimulates oxygen use and delivery.
• It includes respiratory effects.
• It includes a respiratory rate at rest and ventilatory response to hypercapnia and hypoxia
• Affects growth and maturation include normal neurological development and adequate bone tissue formation in the fetus.
• Affects the nervous system: hormonal thyroid difference.
• Inhibits cerebral cortex and cerebellar growth.
• Promotes axon proliferation and dendrite branching as well as synaptogenesis, myelinization, and cell emigration.
• Affects basal metabolic rate and thermogenesis: acceleration of the physiological response to fasting.
• Affects skeletal muscle: energy production and storage.
• Affects the autonomic nervous system and catecholamines.
• Synergistic action with catecholamines and increases metabolism.
• It include: heat production, heart rate, motor creativity, and CNS excitation.
• Potentiates sympathetic nervous system activity.
• Increases the number of beta-adrenergic receptors and generates intracellular second messengers.
• Cardiovascular effects increase cardiac output, heart rate at rest, stroke volume, contraction velocity and potency, and systolic blood pressure.
• It regulates reproductive organs and endocrine glands.
• It includes normal ovarian cycle of follicular development.
• It plays a role in maturation and ovulation.
• It is involved in homologous process of spermatogenesis at the testicular level.
• It includes maintenance of health during pregnancy.
• It includes bone effects and favors endochondral ossification, bone linear growth, and maturation of bone epiphyseal centers.

Thyroid Hormones and Gene Transcription

• Thyroid hormones increase the transcription of a large number of genes.
• Most cells in the body synthesize a high proportion of protein enzymes, structural and transporters proteins, and other substances.
• The net end result is an increase in the general functional activity of the entire organism.

Hormones T3 and T4

• A T3 test measures blood levels of triiodothyronine, a key hormone produced by the thyroid gland.
• The test helps diagnose hyperthyroidism, where the body produces too much thyroid hormone.
• Two forms of T3 exist: T3 bound to a protein and T3 unbound (free).
• A thyroxine (T4) test is a blood test used to diagnose thyroid diseases
• Thyroid hormones produced control how the body uses energy and affects weight, mood, body temperature, muscle strength and more.
• In children, thyroid hormones also affect development.
• A T4 test measures T4 levels in the blood.
• Too much or too little T4 can indicate thyroid disease.
• Two forms of T4 exist: Free T4, the active form that enters tissues, and bound T4, attached to proteins that prevents it from entering tissues.
• T3 and T4 coordinate to regulate energy utilization, control weight, body temperature, muscle strength, and nervous system function.
• Thyroid hormones perform a synergistic action with catecholamines to increase metabolism, heat production, heart rate, motor activity, and CNS excitation. Also regulating oxygen consumption, intervening in protein, lipid, and carbohydrate metabolism.

Thyroid Hormone Regulation

• Regulation occurs on three levels: the hypothalamus-pituitary-thyroid axis controlling hormone concentration.
• The activity of deiodinases regulates hormone amount in target tissues.
• The thyroid gland regulates iodine uptake for hormone synthesis.
• TRH is synthesized in the neurons of the paraventricular nucleus of the hypothalamus, transported to the adenohypophysis via portal vessels. This stimulates TSH synthesis and secretion in thyrotrophs. TSH is released into the blood and stimulates T3 and T4 synthesis and secretion in the thyroid gland, which in turn inhibit TRH and TSH synthesis and release in a negative feedback mechanism.
• Too much thyroid hormone speeds up processes.
• Too little thyroid hormone slows down processes.
• It controls how fast the body uses calories (metabolism), affecting weight gain or loss.
• It can speed up or slow down your heart rate.
• It can raise or lower your body temperature.
• It can speed up or slow down food movement through the digestive tract.
• It affects muscle strength and how quickly the body replaces dying cells.

Thyroid Gland

• Located below the larynx and in front of the trachea.
• It is one of the largest endocrine glands weighing between 15-20g in healthy adults.
• Thyroid gland secretes thyroxine and triiodothyronine, often referred to as T4, T3 respectively.
• The gland secretes calcitonin, a hormone for calcium metabolism.

Hormonal Parameters

• Blood or urine tests can determine levels of various hormones. Hormone levels include:
• Follicle-stimulating hormone (FSH)
• Luteinizing hormone (LH)
• Anti-müllerian hormone (AMH)
• Thyroid stimulating hormones (TSH)
• Triiodothyronine (T3)
• Thyroxine (T4)