How is the thyroid gland controlled by pituitary gland and hypothalmus?

negative feedback.

In a Heating system the heater stays on till it gets hot enough that the thermostat turns it off. Then when it cools down the heater comes back on again.

In the hormone system. What ever gland produces hormones for a while till those hormones activate the other gland to produce different hormones which stop the first gland producing the hormone. Keeping it in a constant state of balance the same way a heating system keeps a house at a constant temperature.

I cant remember which hormones do what in which gland. But I'm sure you can find a flow chart of it if you search "negative feedback hormone regulation" or something like that.

The thyroid gland is under the control of the pituitary gland, a small gland the size of a peanut at the base of the brain (shown here in orange). When the level of thyroid hormones (T3 & T4) drops too low, the pituitary gland produces Thyroid Stimulating Hormone (TSH) which stimulates the thyroid gland to produce more hormones. Under the influence of TSH, the thyroid will manufacture and secrete T3 and T4 thereby raising their blood levels. The pituitary senses this and responds by decreasing its TSH production. One can imagine the thyroid gland as a furnace and the pituitary gland as the thermostat. 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 our picture 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.