Thyroid Hormones Quiz

Thyroid Hormones: Synthesis, Mechanism of Action, and Treatment

• The thyroid gland produces the two major hormones, T3 and T4, which maintain optimal metabolism for normal growth and maturation.
• Hypothyroidism, inadequate hormone secretion, causes bradycardia, cold intolerance, mental and physical slowing, and mental retardation and dwarfism in children.
• Hyperthyroidism, excess hormone secretion, causes tachycardia, cardiac arrhythmias, body wasting, nervousness, tremor, and heat intolerance.
• TSH controls thyroid function, and TSH action stimulates iodide uptake by the gland and oxidation to iodine followed by iodination of tyrosines on thyroglobulin.
• Antibodies to thyroid peroxidase are diagnostic for Hashimoto thyroiditis, a common cause of hypothyroidism.
• T4 is enzymatically deiodinated to T3, which enters the nucleus and attaches to specific receptors, promoting RNA formation and protein synthesis responsible for T4 effects.
• Food, calcium preparations, iron salts, and aluminum-containing antacids can decrease T4 absorption.
• Levothyroxine (T4) is preferred over T3 for hypothyroidism treatment, better tolerated, and dosed once daily, achieving steady state in 6 to 8 weeks.
• Toxicity is related to T4 levels and manifests as nervousness, palpitations, tachycardia, heat intolerance, and unexplained weight loss.
• Thioamides, such as PTU and methimazole, inhibit oxidative processes and condensation of iodotyrosines to form T3 and T4, and PTU also blocks peripheral conversion of T4 to T3.
• A pharmacologic dose of iodide inhibits iodination of tyrosines and hormone release, and iodide is used to treat thyroid storm or before surgery.
• β-Blockers, such as metoprolol or propranolol, are effective in blunting the widespread sympathetic stimulation that occurs in hyperthyroidism and used to treat thyroid storm.