Why Am I So Tired? Thyroid, Testosterone, and Other Causes in Men

You feel tired in a way that sleep does not fix. You are getting through the day, but it takes more out of you than it used to, and by evening you are running on fumes. The first thing a lot of men reach for is "my testosterone must be low." It is a reasonable guess, and low testosterone is a real and common cause of fatigue -- but it is one of several, and treating it without checking the others can leave the actual problem untouched.

Low testosterone is not the only explanation

Fatigue is one of the least specific symptoms in medicine, which is exactly what makes it hard to self-diagnose. Low testosterone can cause it, but so can hypothyroidism, obstructive sleep apnea, depression, anemia, poorly controlled blood sugar, and a handful of nutrient deficiencies. Several of these conditions are also more common in exactly the group of men who tend to assume low testosterone first -- men over 40 carrying extra weight, sleeping poorly, and under more stress than they'd like.

Testosterone does decline naturally with age, and it is common enough that one large study of men 45 and older found roughly 4 in 10 had testosterone levels in the hypogonadal range, though the rate varied depending on other health conditions they had. That is a meaningful number. It is also not the whole picture, since the same study population had high rates of obesity, diabetes, and hypertension -- conditions that independently cause fatigue on their own.

The practical issue is that low testosterone, low thyroid hormone, and sleep apnea can produce nearly identical symptoms: low energy, low mood, reduced motivation, brain fog, and a lower sex drive. You cannot reliably tell them apart by how you feel. You can only tell them apart with the right labs and, sometimes, a sleep study.

Thyroid problems can look exactly like low testosterone

An underactive thyroid (hypothyroidism) slows down nearly every system in the body, and fatigue is one of its most consistent symptoms, often alongside weight gain, feeling cold, constipation, and dry skin. Clinical guidelines for diagnosing and managing hypothyroidism rely on a simple pair of blood tests -- thyroid stimulating hormone (TSH) and free T4 -- which is part of why it is a reasonable first thing to rule out alongside testosterone rather than after it.

Hypothyroidism is also easy to miss because the symptoms build slowly. Men often adjust to feeling a little worse each month without noticing the trend until someone else points it out, or until a lab test makes it obvious. The treatment, when it is truly a thyroid problem, is straightforward: thyroid hormone replacement, dosed and monitored with follow-up labs. It is not something testosterone therapy will fix, and it is not something that resolves with lifestyle changes alone.

Sleep apnea: the cause that often hides in plain sight

Obstructive sleep apnea causes repeated pauses in breathing during sleep, and even when it does not fully wake you up, it fragments the deep sleep your body needs to actually recover overnight. One of the foundational studies on this condition found sleep-disordered breathing to be common in middle-aged adults, and a large share of those affected were undiagnosed. The daytime result is fatigue that does not track with how many hours you spent in bed.

Sleep apnea also has a two-way relationship with testosterone: untreated sleep apnea can lower testosterone levels, and testosterone therapy in some men can worsen sleep apnea. That makes the order of operations matter. If sleep apnea is present and untreated, starting testosterone therapy without addressing it first is a reasonable thing to want to avoid. Snoring, witnessed pauses in breathing, morning headaches, and waking up unrefreshed despite a full night in bed are the symptoms worth mentioning at a visit, since many men never notice the breathing pauses themselves.

Depression, anemia, and vitamin D: the quieter contributors

Depression causes real, physical fatigue, not just low mood -- it is one of the most common causes of persistent tiredness that gets missed because men do not always recognize or report the mood symptoms that usually come with it. National screening guidelines recommend depression screening in primary care specifically because it is common and often under-recognized, fatigue included.

Anemia (low red blood cell count) reduces how much oxygen reaches your tissues, which produces a fatigue that often comes with being short of breath on exertion or looking paler than usual. It shows up on a basic complete blood count, one of the simplest and least expensive tests available.

Vitamin D deficiency is also associated with fatigue and low mood in some people, though the evidence connecting a mild deficiency specifically to energy levels is less consistent than for the conditions above. Clinical guidelines still support checking and correcting vitamin D levels when they are low, since deficiency is common and correction is low-risk.

Blood sugar and metabolic causes worth ruling out

Both high and low blood sugar cause fatigue, and undiagnosed or poorly controlled type 2 diabetes is a common, overlooked cause of the kind of tiredness men attribute to aging or stress. A fasting glucose or A1C test is inexpensive and often included in a baseline metabolic panel, which is part of why it belongs in a fatigue work-up even without other classic diabetes symptoms like excessive thirst or frequent urination.

How we build a fatigue work-up

The honest approach to persistent fatigue is to test broadly before treating narrowly. That usually starts with a detailed history -- sleep habits, mood, diet, alcohol use, medications, and how the fatigue actually feels day to day -- followed by a lab panel that typically includes TSH and free T4, morning total testosterone, a complete blood count, fasting glucose or A1C, and vitamin D. If your history suggests sleep apnea, a referral for a sleep study is part of the plan, not an afterthought.

This matters because the fixes for these causes are completely different from each other. Thyroid hormone replacement does nothing for sleep apnea. A CPAP machine does nothing for iron-deficiency anemia. And testosterone therapy, started before any of this is ruled out, treats a number on a lab report rather than the reason you are actually tired.

If you have been tired for weeks or months and you are ready to find out why instead of guessing, we can start with a telehealth visit and the right labs. Learn more about our approach to men's health. Most major plans are accepted, including Medicare, so getting a real answer does not have to mean paying out of pocket on top of the coverage you already have.