The fetal hypothalamic-pituitary-thyroid system develops from the mother’s pituitary-thyroid axis independently. In the 8 weeks’ gestation, thyroglobulin is produced. By 10-12 weeks’ gestation, trapping of iodine occurs followed by the synthesis of iodothyronines. By the 12 weeks’ gestation, colloid formation and pituitary secretion of thyrotropin, also termed thyroid-stimulating hormone (TSH), occur.
The primary function of the thyroid gland is synthesis of hormones namely, thyroxine (T4) and triiodothyronine (T3). Pituitary thyrotropin regulates thyroid hormone production. TSH synthesis and secretion are stimulated by thyrotropin-releasing hormone (TRH), which is synthesized by the hypothalamus and is secreted into the hypophyseal portal vasculature for transport to the anterior pituitary gland. Serum T4 concentration mediates secretion of both TRH and TSH by negative feedback process.
T4-binding globulin (TBG) a protein, to which circulating T4 is bound. T4 is deiodinated in peripheral tissue to T3, the more bioactive thyroid hormone. T3 carries 3-4 times the metabolic potency of T4, freely enters cells, and binds to receptors of the hormone into the cell nucleus. Thyroid hormone exerts profound effects on the regulation of gene transcription. Some major clinical phenomena of thyroid hormone action include differentiation of the CNS and maintenance of muscle mass. Thyroid hormone also controls skeletal growth and differentiation and metabolism of carbohydrates, lipids, and vitamins.
The recommended dietary allowance of iodine is 40-50 microgram daily in infants, 70-120 micrograms daily for children, and 150 micrograms daily for adolescents and adults. The daily intake in North America varies from 240 micrograms to more than 700 micrograms.
In the thyroid gland, iodide is trapped, transported, and concentrated in the follicular lumen for thyroid hormone synthesis. Before trapped iodide can react with tyrosine residues, it must be oxidized by thyroidal peroxidase. Iodination of tyrosine results in mono-iodotyrosine and di-iodotyrosine formation. Two molecules of di-iodotyrosine combine to form T4, and one molecule of mono-iodotyrosine one molecule of di-iodotyrosine combine to form T3. Formed thyroid hormones are stored within thyroglobulin in the lumen of the thyroid follicle until release. TSH stimulates uptake and organification of iodide as well as liberation of T4 and T3 from thyroglobulin.
Hyperthyroidism is the condition, where the thyroid gland produces too much thyroid hormone (the When the thyroid gland produces too little thyroid hormone, the condition is called hypothyroidism. Thyroid problems can occur at any age; including infants!
Signs of hypothyroidism can include:
Decreased energy,
Decreased growth rate,
Muscle soreness,
Weight gain without increased appetite,
Appearing swollen or puffy,
Constipation or harder stool less often,
Other problems that includes brittle hair and dry skin