Fourth thyroid vein ( Kocher) may be found sometimes, between:
The thyroid is drained by the superior, middle and inferior thyroid veins.
The superior thyroid vein emerges at the upper pole and accompanies the superior thyroid artery. It ends in the internal jugular vein.
The middle thyroid vein is a short, wide channel which emerges at the middle of the lobe and soon enters the internal jugular vein.
The inferior thyroid veins emerge at the lower border of isthmus. They form a plexus in front of the trachea, and drain into the left brachiocephalic vein.
A fourth thyroid vein (Kocher) may emerge between the middle and inferior veins, and drain into the internal jugular vein.

Reference: BD Chaurasia Volume 3, ed 6 page no 143
Superior and middle thyroid vein.
Superior and inferior thyroid vein.
Middle and inferior thyroid vein.
Any of the above.