When T4 and T3 go low, TSH goes high.
The hypothalamus senses low circulating levels of thyroid hormone —Triiodothyronine (T3) and Thyroxine (T4) — and responds by releasing thyrotropin-releasing hormone (TRH).
TRH – Thyrotropin-releasing hormone is a hormone produced in the hypothalamus that stimulates the release of thyroid-stimulating hormone (TSH) from the anterior pituitary gland.
TSH – Thyroid-stimulating hormone (also known as thyrotropin, thyrotropic hormone, or abbreviated TSH) is a pituitary hormone that stimulates the thyroid gland to produce thyroxine (T4), and then triiodothyronine (T3).
Thyroid Gland – The primary function of the thyroid is the production of the iodine-containing thyroid hormones, triiodothyronine (T3) and thyroxine (T4) and the peptide hormone calcitonin. The thyroid hormones are created from iodine and tyrosine. T3 is so named because it contains three atoms of iodine per molecule and T4 contains four atoms of iodine per molecule.
Thyroid Hormones – Thyroid hormones are two hormones produced and released by the thyroid gland — triiodothyronine (T3) and thyroxine (T4). T4 is a pro-hormone and is produced in larger amounts than the biologically active T3.
T3 – Triiodothyronine, the biologically active hormone, is produced in smaller amounts than T4 (thyroxine) by the thyroid gland. Most of the T3 utilized by the body is converted from T4 by removing one iodine atom (T4 minus 1 equals T3). Coversion mainly takes place in the liver and kidneys but also in other tissue and organs. T3 affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate.
Then back to the top to the Hyothalamus and TRH…
HPT axis – the hypothalamic–pituitary–thyroid axis is a negative feedback loop that senses low circulating levels of thyroid hormones (T3 and T4). When the hypothalamus senses low levels it sends TRH to the pituitary gland, which then sends TSH to the thyroid gland to stimulate production of thyroid hormones.
When T3 and T4 levels go down, TSH goes up.
When T3 and T4 levels go up, TSH goes down.
Detailed information from Wikipedia:
TRH – Hypothalamus
TSH – Pituitary Gland
T3 – Triiodothyronine
HPT axis – hypothalamic–pituitary–thyroid axis