quoting, for example, (Lubavich) R Dubov Laws of Yichud ch. 10, see for more there
In general, a man should try to visit a male doctor, and a woman, a female doctor. However, if no doctor of the same gender is available, it is permitted to be treated by a doctor of the opposite gender.
meeting takes place in a public office or clinic during office hours
door to the doctor’s office is open slightly or closed but not locked, and other members of staff may enter at any time.
Tzitz Eliezer Vol. 6:40:12 writes that a woman may enter a doctor’s office for an examination if the door is closed but not locked (as long as there are three people in the waiting room, or else a man and his wife), for then it is considered a Pesach Posuach Lirshus Horabim. However, Nishmas Avraham 22:1 quotes Rabbi S. Z. Auerbach zt’l: “Rav Auerbach zt’l also told me that as long as a nurse, patient or any other person can at any time enter the examination room, whether as part of his or her job or whether in error, there is no transgression of Yichud.” The implication is that this Yichud situation is permissible even if there is nobody in the waiting room at the time, since the appointment is within office hours and someone may enter at any moment, hence creating a Pesach Posuach. This is also evident from the following Responsa from Igros Moshe Even HoEzer Vol. 4:65:1 who writes: “As regards a woman who needs to be examined by a doctor while undressed (even though there is another person in another room of the doctor’s offices, and it is not customary for anyone to enter his examination room, nevertheless,) it is a daily occurrence that even the wives of Torah scholars go alone to the doctor and are secluded with him in his office. This lenient behavior has become widespread because the doctor, be he a Jew or a non-Jew, is intent on his work, removing from him any lustful feelings. Even when he finishes his examination he cannot spend extra time with her, for there are other patients whom he has scheduled for the following time slot. And even if the next patient has not yet arrived, he fears that he or she may arrive soon. If she is the last patient for the day and he knows that no other patients are expected, one may rely on the presence of an employee in his office, for should he spend more than the usual amount of time with the patient, this will arouse suspicion. The doctor, Jew or non-Jew, will be careful that no suspicion is cast on his name that might harm his practice. However, she cannot be permitted to be secluded with him when there is no one else in the office and when no other patient is expected, and her son, daughter or husband must accompany her to such an appointment. But, if she did go to the appointment alone at such a time and transgressed the prohibition of Yichud, she is not forbidden to her husband, for Yichud alone is not sufficient to forbid a woman to her husband.” See also Minchas Ish 18:4.
Shevet Halevi (Shiurei Shevet Halevi Hilchos Niddah p. 278, Responsa Vol. 4:167) summarizes the guidelines of visitation by women to medical personnel:
She should minimize as much as possible examination by male personnel.
Wherever possible, she should request a female doctor/nurse.
When a female doctor is not available or the male doctor is better qualified, it is permitted for her to be examined by a male.
Under no circumstances may a woman be examined by any medical personnel who are known to be promiscuous.
She should only visit a doctor during office hours when others are present in the office.
Whenever possible, she should be accompanied by another woman.
The doctor should leave the door of the office slightly ajar, or at least closed but not locked.
Non-observant medical personnel may advise a woman to do something in violation of halachah. Therefore, women should always take medical advice in conjunction with a competent Rav.
She should not engage in prolonged conversation or joke with the doctor, but rather just focus on the medical issue at hand.
As regards her husband accompanying her to the office, Shevet Halevi writes that if the husband will have to wait in a waiting room full of women (who may be prutzos), then this accompaniment for the purpose of mitigating the Yichud situation may be detrimental in terms of tznius. (However, it seems to this author that if the visit is to a private clinic in the doctor’s home or private offices it is preferable for her husband to attend.)