I'll try and write a little summary for you. It is all part of a self regulating feedback system.

The function of the thyroid gland is to take iodine, and convert it into thyroid hormones: T3 and T4. These control metabolism of all the cells in your body, hence all the side effects rayray mentioned.

The thyroid gland is under the control of the pituitary gland, 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. Similarly, if the levels are too high, then the pituarty stops secreting TSH. (FSH is also produced by pituatry, but i am unsure of its mechanisms. The pituitry secrets many of hormones)

All hormone secretions from the pituitary gland are regulated by the hypothalamus. The hypothalamus is part of the brain and releases a hormone (TRH) which tells the pituitary gland to stimulate the thyroid gland (ie release TSH). (Similarly, the hypothalamus releases a different hormone to tell the pituitary to release FSH)

Summary: HYPOTHALAMUS-(TRH)---------->PITUITARY-(TSH)--->THYROID-(T3 & T4)--------> feeds back to regulate secretions.

I know i havn't really answered your qns about FSH, it would seem to me that it is part of a separate system but using the same pathways. But that is me making assumptions, sorry i cant give you a more definative answer.
Let me know if i can explain it better?! I cut and paste a bit from an old assignment, hope is makes some sense!

Hope the test results are good tomorrow.