Thyroid hormones are like heat. When the heat gets back to the thermostat, it turns the thermostat off. As the room cools (the thyroid hormone levels drop), the thermostat turns back on (TSH increases) and the furnace produces more heat (thyroid hormones).
The pituitary gland itself is regulated by another gland, known as the hypothalamus (shown in the picture above in light blue). The hypothalamus is part of the brain and produces TSH Releasing Hormone (TRH) which tells the pituitary gland to stimulate the thyroid gland (release TSH). One might imagine the hypothalamus as the person who regulates the thermostat since it tells the pituitary gland at what level the thyroid should be set.
Chronic emotional or physiologic stress can cause the significant reduction of T4 into the cells of the body while the pituitary is unaffected. A study published in the Journal of Clinical Endocrinology and Metabolism studied the effect of adding serum from different groups of individuals to cell cultures and measured the amount of T4 uptake from the serum into the cell. The study found that the serum from those with significant physiologic stress inhibited the uptake (transport) of T4 into the cell while the serum from non-physiological stress had no effect, demonstrating that serum T4 levels are artificially elevated in physiologically stressed individuals and that serum T4 and TSH levels are poor markers for tissue thyroid levels in stressed individuals (4).