Normally, T4 and T3 exert negative feedback on TSH secretion by pituitary in two ways: Block the secretion of TSH by pituitary directly --> major action Block the secretion of TRH by hypothalamus (TRH stimulates the secretion of TSH by pituitary) In primary hypothyrodism, T4 and T3 are not produced or produced in low concentration by thyroid. This results in abolition of negative feedback on TSH secretion. Thus, there is elevated TSH and pituitary can become hyperplastic to produce more TSH. In pituitary cause of hypothyroidism (secondary hypothyroidism) both TSH and Thyroid hormone (T4, T3) are low as pituitary does not secrete TSH and TSH is the major stimulation for production of T4 and T3. About option d TSH resistance can also produce same picture i.e. raised TSH and low T4 with pituitary swelling. There is thyroid insensitivity to TSH which results in hypothyroidism (1T4 and T3). Because of reduced thyroid hormone feedback, TSH is markedly elevated. However, Amongst the given options best option is primary hypothyrodism because TSH resistance is a very rare condition and further T4 levels are normal in Mild and moderate (Partial) TSH resistance.
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