How secretion is controlled

As blood concentration of the thyroid hormones lower, secretions of the thyroid releasing hormone 

(From hypothalamus) and thyroid stimulating hormone (from pituitary gland) which stimulates secretions of the T3 and T4

High Blood concentrations of thyroid hormones inhibits the release of TSH, and TRH

*hormones released by negative feedback

The release of calcitonin also works in a similar way. Raised Blood concentrations of Calcium and Phosphate ions stimulate secretion, while low blood concentrations equals suppressed secretion