Usually thought to be an autoimmune process whereby the thyroid gland increases output of T4 and T3. This produces direct feedback to the hypothalamic and pituitary regions whereby TSH then is "turned down" and suppressed.  

Usually thought to be an autoimmune process whereby the thyroid gland increases output of T4 and T3. This produces direct feedback to the hypothalamic and pituitary regions whereby TSH then is "turned down" and suppressed.  

As per the study (Toft AD. Clinical practice. Subclinical hyperthyroidism. N Engl J Med 2001;345:512-516) the decreased TSH level seen in SCH results from the pituitary’s response to minor elevations in serum or tissue T4 and T3 levels.

the state of hyperthyroidism in fetuses

or early neonates to passive transfer of maternal T4 from a mother

with hyperthyroidism or thyrotoxicosis through placenta and in

mother’s milk. The decrease in serum TSH level is attributed to

negative feedback effect of the excess circulating THs levels on

the anterior lobe of pituitary gland. The lowering in serum TSH

level, in the current study, may play a crucial role in the atrophy

of the thyroid gland due to loss of stimulatory effect of this hormone

on the gland. These suggestions are supported by Fisher et

al. (2000) and Higuchi et al. (2005) who reported that the exposure

of the fetal hypothalamic-pituitary–thyroid system to a higherthan-

control thyroid hormone (T4) concentration might impair its

physiologic maturation, because there is a continuous significant

decrease in the TSH/fetal T4 ratio during the development.

It indeed occurs in response to negative feedback between thyroid hormones and pituitary TSH.

The pathophysiology of various disorders/ diseases (Graves' disease, toxic thyroid adenoma, toxic multi-nodular goiter, pituitary adenoma, thyroiditis etc.) and various physiological/ pharmacological influences involving a variety of alterations (different for different conditions)  establishing negative feedback between T3, T4 and production of TSH  may cause hyperthyroidism.