Doctor's pens and pen sets come in many variants, the common denominator being an incorporated thermometer. The earliest doctor's pens were hard rubber eyedropper-fillers with an internal compartments for the thermometer, which was withdrawn from the end of the barrel opposite the nib. Parker's version is the most famous, though surviving examples are few. The example shown below was made by Laughlin.
Later on, OMAS offered their own doctor's pen, similarly configured. None of these all-in-one pens seem to have caught on, but by the 1920s and 1930s, doctor's sets were introduced which paired a standard fountain pen with a matching thermometer case, a pencil often forming part of the ensemble. The Waterman 52 set below is an early example.
After the widespread adoption of celluloid for pen manufacture, these sets were often made in white, though not always. The 1930s Vacumatic doctor's set below used the standard silver pearl laminate.
Doctor's pens are quite scarce, and those in light colors are usually found with discoloration and cracks. Nurse's pens and sets, however, are considerably more common; most often seen are the 1940s Watermans in ivory pearl with either red or black cap crowns, and the 1950s Esterbrook purse pens in white. These are smaller pens, whereas doctor's pens are normally full-sized. White pens made for confirmation gifts are also typically small, and not to be confused with either doctor's or nurse's models.