Thyroid Hormone Regulation

Control of Thyroid Hormone Secretion

• TSH promotes the release of thyroid hormones (T3 and T4) by the thyroid gland in response to low levels of thyroid hormones and increased metabolic rate.
• TRH in the hypothalamus stimulates the release of TSH from the anterior pituitary.
• TSH receptors are located in the anterior pituitary.
• The thyroid follicle cells are the control center, and most body cells are the effectors.
• The response to TSH is an increased metabolic rate and increased levels of thyroid hormones (T3 and T4).

Laboratory Investigations

• Results are typically available within 48 hours, but in cases of thyrotoxic crisis or myxoedema coma, results are available within 2 hours.
• Blood tests measure TSH, FT4, and FT3 to establish if there is thyroid dysfunction.
• Auto-antibody tests are used to elucidate the cause of thyroid dysfunction.

Thyroid Diseases

• Thyroid diseases are especially common in women, with a prevalence of 3-5% in the community.
• Thyroid diseases can be caused by excessive or inadequate amounts of thyroid hormones, inflammation or infection of the thyroid gland, or benign or malignant growth.
• Thyroid diseases are easily treatable with an excellent long-term outcome.

Hypothyroidism and Hyperthyroidism

• Hypothyroidism is characterized by coarse and dry hair, dry skin, puffiness of the face, swelling around the eyes, tiredness and lethargy, bradycardia, weight gain, and intolerance to cold.
• Hyperthyroidism is characterized by lid retraction, sweating, anxious look, goitre, palpitations, intolerance to heat, and weight loss.

Hypothyroidism (Under-Activity)

• Causes of hypothyroidism include low T4, low T3, and high TSH (primary), low T4, low T3, and low/normal TSH (secondary), thyroiditis, certain drugs, congenital causes (leading to cretinism), iodine deficiency, and disorders of the pituitary gland or hypothalamus.

Hyperthyroidism (Over-Activity)

• Causes of hyperthyroidism include high T4, high T3, and undetectable TSH, T3 toxicosis, thyrotoxicosis, Graves' disease, toxic nodular goiter, thyroiditis, drug-induced hyperthyroidism, gestational hyperthyroidism, struma ovarii, and thyroid-secreting pituitary adenoma.

Goitre

• Goitre is an enlarged thyroid gland, which can be small, large, or huge.

Thyroid Hormones

• TSH controls thyroid cell growth and synthesis, and secretion of thyroid hormones.
• TSH secretion is inhibited by T4 and T3 (negative feedback).
• Thyroid hormones produced by the thyroid gland regulate metabolism and growth.
• TSH is synthesized by the anterior pituitary and composed of two subunits: α and β.
• Normal range of TSH is 0.4-3.8 mU/L.

Thyroid Hormones - Functions

• Thyroid hormones increase basal metabolic rate (oxygen consumption and heat production), cardiac output, and amplify the catecholamine effect.
• They increase hypercapnic drive in the respiratory center, production of erythropoietin, bone turnover, and hepatic gluconeogenesis and glycogenolysis.

Circulating Concentration

• T4: 60-170 nmol/L, involved in digestion, brain development, and bone health.
• T3: 0.8-2.7 nmol/L, involved in 20% of thyroid hormone production, and 80% is produced from the conversion of thyroxine.

The Production of Thyroid Hormones

• Iodide trapping: The thyroid gland gathers iodine from the blood.
• Synthesis of thyroglobulin: A protein called thyroglobulin is produced.
• Oxidation of iodide: Iodine is converted to its active form.
• Iodination of tyrosine: Iodine is attached to tyrosine (an amino acid) in thyroglobulin.
• Coupling of T1 and T2: Iodinated tyrosine molecules link together.
• Pinocytosis and digestion of colloid: The thyroglobulin containing colloid is engulfed and broken down.
• Secretion of thyroid hormones: T4 and T3 are released in the blood stream.
• Transport of thyroid hormones in the blood: T4 and T3 travel through the blood stream carried by proteins.

Understand how TSH promotes the release of thyroid hormones and the control mechanisms involved in regulating thyroid hormone secretion. Learn about the stimulus, controlled conditions, receptors, control center, effectors, and response in thyroid hormone regulation.