From “Ask the Doctor” column of American Sleep Apnea Association Newsletter:

Why does sleep apnea sometimes lead to frequent urination at night?
Nocturia, or nocturnal urination, is sometimes a symptom of sleep apnea. In fact, nocturia in younger adults-who are less likely to have other medical causes of nocturia-is a strong indicator of sleep apnea. While the precise relationship has not been thoroughly studied, it appears that the most likely reason patients with untreated sleep apnea have more frequent urination at night is related to the increased pressure in the right side of the heart. This increased pressure is usually the result of low oxygen levels in the bloodstream caused by the apnea events: when oxygen levels fall, the heart works harder to get oxygen to the brain.

An increased pressure in the heart is a sign that there is too much liquid in the body: when the heart receives the stimulus of the increased pressure, higher levels of a hormone called atrial natriuretic peptide (ANP) are secreted from the heart. ANP is a diuretic that is associated with the increased need to urinate.

When sleep apnea is effectively treated, nighttime urination can be dramatically reduced. Studies have shown that ANP levels in patients with untreated sleep apnea are increased and levels reduced in patients using CPAP effectively. Remember, not all causes of frequent urination are related to untreated sleep apnea; prostrate problems, for example, may cause increased need for urination. Discuss any concerns you may have with your doctor.
Dr. Patrick Strollo, Jr., MD is the Medical Director of the Pulmonary Sleep Evaluation Laboratory at the University of Pittsburgh Medical School. August-September 1996
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More than a year ago, I was diagnosed with sleep apnea (120 events per hour) but even with CPAP felt tired. I was then prescribed a thyroid and have lost thirty pounds and am now fine without CPAP. What is the relationship between sleep apnea and the thyroid?
Hypothyroidism: Several studies have found that the risk of sleep apnea is increased in patients who have hypothyroidism. However, the majority of people with sleep apnea are NOT hypothyroid, so most sleep clinicians don't check for it routinely.

The thyroid produces hormones that regulate metabolism. When this gland is underactive, the condition is called hypothyroidism (when overactive, hyperthyroidism). Hypothyroidism appears to predispose to sleep apnea in at least three ways: it leads to obesity; it causes decreased drive to breathe and swelling of the soft tissues in the airway (including the tongue), thereby contributing to obstruction; and it may result in weakness of the breathing muscles of the upper airway. Macroglossia or big tongue is a feature of longstanding untreated hypothyroidism and may not go away with treatment.

Thyroid function blood testing is not routine in all patients who have sleep apnea but may be ordered in patients who appear hypothyroid. Some clinical signs of hypothyroidism are lowered voice, intolerance of cold, hair loss, slow pulse rate, constipation, and decreased reflexes. People who have sleep apnea but no obesity, anatomic abnormality, or family history to explain sleep apnea may also be tested.

Thyroid hormone replacement often but not always results in improvement of sleep apnea in hypothyroid patients. However, normalization of thyroid status with replacement therapy can take months and does not always result in complete cure of sleep apnea. Patients with sleep apnea who are getting thyroid replacement should continue to use CPAP until repeat clinical evaluation documents that their steep apnea is improved enough to do without it. They should make this decision in conjunction with their physician.
Barbara Phillips, MD is the director of the Sleep Disorders Center at Columbia Hospital in Lexington, Kentucky. October-November 1997
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How do allergies affect sleep apnea and CPAP?
Allergies, particularly those that result in allergic rhinitis (inflammation of the nasal passages) or other upper airway inflammation, may promote or exacerbate obstructive sleep apnea (OSA). These allergic reactions, whether to pollens, dust, or other environmental allergens, also tend to interfere with nasal CPAP. Some patients may find they cannot tolerate CPAP until they are symptomatically relieved, but stopping therapy is not a wise solution.

In any case, treatment strategies with OSA are designed to decrease upper airway inflammation whenever possible. Systemic antihistamines, decongestants, and nasal steroid sprays are mainstays of therapy in this regard.

However, antihistamines can cause drowsiness, and patients with OSA or other causes of daytime sleepiness should be aware that they should exercise caution when using them. It is generally recommended that such medications be taken at night, and shorter-acting antihistamines are preferred. Topical decongestants are generally best to avoid, as they tend to cause rebound congestion even after only a short period of use.

In some cases, positional therapy may help: when congestion is worse, avoid the supine position (sleeping on the back) or sleep with the back elevated from the waist up, using foam wedges. Talk to your prescribing physician so that a rational response can be developed.
Robert Basner, MD, Director of the Center for Sleep and Ventilatory Disorders at the University of Illinois at Chicago College of Medicine. April - May 1997