Feeling Unlike Yourself? Other Causes a Doctor Can Check

Many conditions look alike from the inside.

By Neil Solomon · 2 min read

Picture a couple named Henry, 55, and Joan, 53. Henry has felt tired and gray for months. Joan says it must be his hormones. Henry says it must be age. Neither has asked a doctor to look for anything else.

Many conditions look alike from the inside. Here are the ones with the strongest evidence.

Sleep

Sleep apnea is common in midlife. In one large US study, 17 percent of men aged 50 to 70 had moderate or severe sleep apnea, and 9 percent of women. Each step up in severity went with 1.8 times the odds of developing depression. People who sleep poorly for other reasons, called insomnia, were about two to two and a half times as likely to become depressed later.

Thyroid

An underactive thyroid affects about 4.6 percent of the US population in a large survey from the 1990s, most of it mild. The NHS lists tiredness, low mood, weight gain and feeling cold among the symptoms. A blood test settles it.

Alcohol

Research pooling many studies found that alcohol use disorder and major depression each roughly double the odds of the other. The link runs both ways, and it often goes unnoticed because the drinking feels like relief.

Depression itself

In 2021, 8.3 percent of US adults had a major depressive episode, 10.3 percent of women and 6.2 percent of men. Depression has well-tested treatments.

Vitamins

The NHS lists tiredness for iron-deficiency anemia, and fatigue with mild depression or anxiety for vitamin B12 deficiency. In a large trial of more than 18,000 adults, vitamin D3 did not prevent depression.

Medicines

Two common ones often get blamed. A review of 15 trials found beta-blockers did not raise depressive symptoms, though they were linked to a small increase in fatigue. A 2026 analysis of 123,940 people in blinded trials did not support statins causing depression.