Traditionally, which is where the planetary rulerships come from, there was no thyroid per se. According to Dariot, Venus rules the throat, not Mercury. However, Culpeper and Lily both give the throat to Mercury. I'd say Taurus rules the throat, as well as the natal second, so if either are afflicted then you can look that way. If you are using your natal chart, that might be the way to go. Horary would probably be a short cut for you.
I am turning more and more towards this theme wherever dual/outer rulership of a sign is concerned, yet astrologers still express different views regarding the thyroid gland.
If Taurus is the
region of 'the neck', then Venus as a
planetary function and working through its rulership of the sign has influence over the thyroid gland. Iodine produced by the gland is said to be ruled by Venus. General 'hormone balance' is also a speciality of Libra, Venus's 2nd rulership.
Yet if the neck is seen as the region to, and including the Mercury-ruled trachea leading into the Gemini-ruled respiratory system, Mercury as ruler of Gemini is the alternative.
Neither Gemini or Virgo have association with hormone balance.
I check with Google in seeking verification and/or answers to astrological queries regarding the anatomy. Picture diagrams are the best, of course. They show that the thyroid gland in the throat is situated
above the trachea, which would seem to verify its rulership through Taurus and Venus.
The thyroid gland is said to have associations with the intellect, but whether this is in the mental or emotional sense is not clarified.
I have had a thyroid problem for many years. I have a Mercury-Venus conjunction in Taurus in the 9th house (health matters). I can be rather 'sluggishly lazy' towards that which does not physically capture my interest, yet my mind is always over- than under -active!
It would be interesting for learning purposes to see the O.P's (anonymous?) natal chart to check for any correspondences.