Forty-seven-year-old Jeff Blakeman applied for Social Security disability benefits in July 2003, claiming a disability onset date of October 1,1997. He suffers from a genetic heart condition that allegedly causes disabling fatigue, dizziness, shortness of breath, and arrhythmia. The administrative law judge (ALJ) held a hearing on December 13, 2004, and found *879Blakeman not disabled. The Appeals Council denied review. Blakeman filed this action for judicial review. The district court1 upheld the final agency decision. Blakeman appeals raising a single issue-whether substantial evidence on the administrative record as a whole supports the ALJ’s finding that Blakeman’s subjective complaints of disabling fatigue due to his heart condition are not entirely credible.
Fatigue is one of the subjective symptoms that must be considered when, as in this case, it is cited by a claimant as a cause of his disability and a medically determinable impairment “could reasonably be expected to produce” that symptom. 20 C.F.R. §§ 404.1529(b), 416.929(b). Our decision in Polaski v. Heckler, 739 F.2d 1320, 1322 (8th Cir.1984), established the factors an ALJ should consider when evaluating the credibility of subjective complaints. Because “questions of fact, including the credibility of a claimant’s subjective testimony, are primarily for the [Commissioner] to decide,” our review is limited to determining whether the ALJ considered all the evidence relevant to Blakeman’s complaints of disabling fatigue and whether that evidence contradicted his account sufficiently that the ALJ could discount his testimony as not entirely credible. Benskin v. Bowen, 830 F.2d 878, 882 (8th Cir.1987). After careful review of the record, we affirm.
I.
Blakeman and members of his family suffer from hypertrophic obstructive car-diomyopathy, a hereditary heart condition also known as idiopathic hypertrophic sub-aortic stenosis or IHSS. The administrative record includes a medical website’s description of the condition submitted by his treating cardiologist, Dr. Kelly Vaughn-Whitley. The major abnormality is an excessive thickening of the heart muscles, which tends to restrict the amount of blood the heart can hold and eject with each contraction. The condition is usually apparent by the late teens and stable thereafter, although some persons experience worsening symptoms in later life. Symptoms include shortness of breath, chest pain, palpitations, lighthead-edness, and blackouts, symptoms which often' have other causes. Blakeman, for example, is obese and was a heavy smoker until he reduced his smoking to a few cigarettes per day in recent years. The effects of IHSS vary greatly. Those with severe symptoms face the risk of heart block and even sudden death. IHSS is commonly treated with medications, a pacemaker implant, and in severe cases heart surgery.
After Blakeman received a pacemaker implant in early 1995, his doctor advised that he could return to normal activities. He returned to his sporadic work as a truck driver, maintenance worker, and carpenter, working as much as ten hours per day at these jobs after the alleged disability onset date, October 1, 1997. He did not report for recommended periodic checks of his pacemaker until September 1998, when his brother died, possibly from a pacemaker malfunction. At that time, he told the doctor he did not have any particular problems. From July 1999 to October 2000, he performed strenuous mobile home warranty repair work for his brother’s business. The ALJ noted that he stopped this work “for reasons not related to his allegedly disabling impairments.”
At a July 2001 disability examination, Blakeman noted “activity intolerance” and *880complained of “frequent episodes of feeling tired and having dizzy spells.” The examining physician, Dr. Paul Johnson, opined that Blakeman “could sit or stand for up to 8 hours a day for work, but he is not capable of any lifting or carrying activities” because of the “great risk” of sudden death from his heart condition. Dr. Johnson saw Blakeman again in October 2001 when he complained of low back pain after “working on his vehicle.” Dr. Johnson prescribed rest and medications, observing that Blakeman was a “[h]ealthy male in no acute distress.”
In September 2001, Blakeman returned to Dr. Vaughn-Whitley complaining of extreme fatigue that increased with activity and episodes of dizziness, blurred vision, heart palpitations, and near black-outs. He reported that “he is unable to maintain a job” because of fatigue and other symptoms. The clinic prescribed medications and reprogrammed his pacemaker in November. In May 2002, Blakeman reported that his dizziness and palpitations improved after the pacemaker was reprogrammed and that he exercised a half-hour each morning. His pacemaker was at the end of its useful life. It was replaced with an AICD device because of his heart condition. The physician’s report described him as a “[w]ell-developed, obese, tired-appearing male currently in no apparent distress.” When Dr. Vaughn-Whitley checked the AICD device in July 2002, Blakeman had “no particular complaints.” In November 2002, when he reported episodes of dizziness at the end of the day, Dr. Vaughn-Whitley adjusted the AICD. In August 2003, Blakeman reported fatigue, dizzy spells with temporary loss of vision, shortness of breath, and trouble sleeping. Dr. Vaughn-Whitley prescribed medications and again adjusted the AICD.
In November 2003, in response to a request from Blakeman’s attorney, Dr. Vaughn-Whitley opined that Blakeman’s heart condition limited him to “episodes” of less than fifteen minutes walking, fifteen minutes standing, and thirty minutes sitting. The letter did not state how many such episodes Blakeman could tolerate in an eight-hour day. That same month, Dr. Larry Vander Woude performed a residual functional capacity assessment in connection with Blakeman’s disability claim. Dr. Vander Woude opined that Blakeman could occasionally lift twenty pounds, frequently lift ten pounds, stand or walk six hours in an eight-hour workday with frequent breaks, and sit six hours in an eight-hour workday. Dr. Vander Woude noted Dr. Vaughn-Whitley’s opinion that Blake-man’s capacity for walking, standing, and sitting was more limited because of his heart condition. A third physician performed another residual functional capacity assessment in February 2004, agreeing with Dr. Vander Woude as to Blakeman’s capacity to walk, stand, and sit and opining there was “no basis” for Dr. Vaughn-Whitley’s more limited opinion.
In August 2004, Blakeman arrived a day early for an appointment with Dr. Vaughn-Whitley. He refused to reschedule the appointment, declaring that it was unnecessary because he had no problems and that he would come back in six months or a year. However, he visited Dr. Vaughn-Whitley in October 2004 complaining of dizziness, shortness of breath with orthopnea,2 and dyspnea3 from walking 300 feet to WalMart. Dr. Vaughn-Whitley suspected his heart condition was *881the cause of these symptoms and reprogrammed his AICD pacemaker.
At the December 2004 hearing, Blake-man testified that fatigue from his heart condition is the primary reason he feels he is disabled. He helps with household chores and grocery shopping, drives a car infrequently for short distances, reads and plays cards for hobbies, and attends church but not regularly. He naps one or two hours each afternoon, has dizzy spells most days, and is short of breath after walking twenty or thirty feet.
The other witness at the hearing was vocational expert William Tysdal. The ALJ recognized that Blakeman’s heart condition is a severe impairment and therefore a vocational expert was needed to help complete steps four and five of the evaluation process, the determination of whether Blakeman has the residual functional capacity (“RFC”) to perform his past relevant work, and if not, whether the Social Security Administration met its burden to show that Blakeman has the RFC and the skills needed to perform “other work [that] exists in significant numbers in the national economy.” See 20 C.F.R. §§ 404.1520, 404.1560, 416.920, 416.960.
The ALJ asked Tysdal whether jobs exist in significant numbers in the national or regional economies for a worker of Blakeman’s age, education, and prior work experience who has the following RFC: stand or walk for four hours and sit for at least four hours in an eight-hour day if allowed to alternate between these positions; lift and carry up to fifteen pounds occasionally and ten pounds frequently; never climb ladders or scaffolds; occasionally climb stairs, balance, kneel, crouch, and crawl; no concentrated exposure to extreme cold; and no work in places that could damage a pacemaker. Tysdal responded that this hypothetical worker could perform certain unskilled sedentary jobs such as food and beverage order clerk or call-out operator. The ALJ then asked:
Q. If we assume that this hypothetical worker needs to lie down during the day ... as you heard from [Blakeman’s] testimony, would any of these jobs exist?
A. No.
Q. Would any other jobs exist?
A. No.
Blakeman’s attorney then asked Tysdal whether someone with the limitations set forth in Dr. Vaughn-Whitley’s November 2003 letter could perform jobs existing in significant numbers in the national economy. Tysdal responded by noting a significant ambiguity in that letter:
A. ... [I]f I can just kind of summarize what ... my feelings are about the limitations, 15 minutes walking, 15 minutes standing, which would be a total of 30 minutes walking and standing, and then 30 minutes sitting. If the individual could perform that throughout an eight-hour workday, then ... the occupations I identified would be within these limits.
Q. Okay. And it’s unclear from the letter, to you, whether that’s what she’s saying or not ... ?
A. Well, if you add it up, it looks to me like 30 and 30: 30 minutes walking and standing, 30 minutes sitting ... it’s per episode. There’s no limitation per day.
On December 15, 2004, two days after the hearing, Dr. Vaughn-Whitley wrote another letter to Blakeman’s attorney that is part of the administrative record. She opined that Blakeman has “multiple medical problems” and “his fatigue could very well be related to any one of these conditions.” She then addressed the question of Blakeman’s functional capabilities:
Again, as stated in a letter dated November 5, 2003 Mr. Blakeman is limited *882as to his ability to perform certain activities. Especially of concern is his ability to perform any job in which he has to be available on a consistent day-to-day basis for long periods of time.
He is able to walk for less than 15 minutes per episode. In fact, at his last visit in October 2004, he informed me he was unable to walk 300 feet without having shortness of breath.
Standing is limited to 15 minutes per episode. Sitting is limited to 30 minutes per episode.
In his February 16, 2005, post-hearing opinion, the ALJ credited Dr. Vaughn-Whitley’s opinions as the treating cardiologist but found that Blakeman’s “statements concerning his impairments and their impact on his ability to work are not entirely credible.” The ALJ then found that Blakeman has the residual functional capacity described in the hypothetical to vocational expert Tysdal: standing or walking four hours and sitting at least four hours in an eight-hour work day (consistent with Dr. Johnson’s 2001 disability evaluation and a somewhat greater limitation than Dr. Vander Woude’s November 2003 RFC assessment), provided he is able to alternate between standing, walking, and sitting (consistent with Dr. Vaughn-Whitley’s opinion). Based on this functional capacity and the testimony of vocational expert Tysdal, the ALJ found that Blakeman could not perform his past relevant work but retains the capacity to perform other unskilled sedentary jobs such as order clerk or call-out operator and is therefore not disabled.
II.
On appeal, Blakeman’s sole argument is that the ALJ wrongly found his subjective complaint of disabling fatigue not entirely credible. This attacks a subsidiary finding. The ALJ’s critical finding, based on medical opinions in the record, was that Blakeman had the RFC to stand or walk for four hours and sit for at least four hours in an eight-hour day, if allowed to alternate between these positions. Vocational expert Tysdal testified that, if Blakeman has that RFC, jobs exist in significant numbers that he can perform. Many workers suffer from fatigue but are able to work, just as many people suffer from chronic pain that is not disabling. The issue is not whether Blakeman’s heart condition is fatiguing, it is whether his fatigue is disabling. As in Moad v. Massanari, 260 F.3d 887, 892 (8th Cir.2001), the ALJ found Blakeman’s complaints of fatigue “credible only to the extent consistent with [the ALJ’s] RFC findings.” That must be our focus.
Blakeman testified that he naps one or two hours each afternoon. The ALJ used this testimony to focus the fatigue inquiry in questioning the vocational expert. Tys-dal responded that there are no jobs available for someone whose RFC is limited by a need to lie down during the work day. The issue is not whether Blakeman was credible in testifying that he naps each weekday afternoon he is not working. The issue is whether his heart condition compels him to nap each afternoon.
Blakeman conceded at oral argument that it is “very difficult” to overcome the ALJ’s credibility finding for the period beginning with the October 1997 onset date and extending into 2003 given the strenuous work Blakeman sporadically performed until October 2000, Dr. Johnson’s disability evaluation in July 2001, and the medical reports showing that Blake-man’s IHSS condition was stable and being effectively controlled by medications and a pacemaker. However, Blakeman argues, even if his complaints of disabling fatigue were not credible in those early years of the alleged disability period, the medical evidence of disabling fatigue be*883ginning in August 2003 is “extremely powerful” and compels the conclusion that the ALJ’s credibility finding for this later period is not supported by substantial evidence.
As Blakeman’s attorney recognized at the hearing, the medical evidence of disabling fatigue is based upon Dr. Vaughn-Whitley’s November 2003 letter, which opined that Blakeman is only capable of walking less than 15 minutes, standing 15 minutes, and sitting 30 minutes “per episode.” When asked whether someone with those limitations could perform jobs existing in significant numbers in the national economy, Tysdal responded yes, if the ambiguous reference to “episode” meant a cycle of alternating positions that could be repeated over the course of a work day. After the hearing, Blakeman submitted a post-hearing letter from Dr. Vaughn-Whitley that simply reiterated the limitations set forth in her November 2003 letter. In other words, after identifying at the hearing an ambiguity perceived by the vocational expert to be critical to the claim of disabling fatigue, Blakeman submitted a post-hearing letter from the physician that either did not clarify the ambiguity or confirmed the vocational expert’s interpretation that the limitation as to walking, standing, and sitting during a sixty-minute episode was consistent with Blakeman being able to stand or walk a total of four hours and sit a total of at least four hours during an eight-hour work day, provided he can alternate positions.
In his post-hearing opinion, the ALJ made the RFC finding and then based his disability determination on Tysdal’s testimony logically interpreting Dr. Vaughn-Whitley’s opinion as consistent with that RFC. There is other evidence in the record supporting the implicit finding that his IHSS condition has not worsened in recent years to the point that it caused disabling fatigue in late 2003 and 2004, such as Dr. Vaughn-Whitley’s advice in April 2004 that Blakeman needed to become more physically active, Blakeman’s refusal to reschedule an appointment with Dr. Vaughn-Whitley in June 2004 because it was unnecessary, and his continuing to smoke while complaining of shortness of breath. After careful review of the administrative record, we conclude that the ALJ’s finding discounting Blakeman’s complaints of fatigue to the extent inconsistent with the finding of his RFC is “supported by good reasons and substantial evidence.” Gonzales v. Barnhart, 465 F.3d 890, 894 (8th Cir.2006). Accordingly, the judgment of the district court is affirmed.
. The HON. RICHARD H. BATTEY, United States district judge for the District of South Dakota
. Orthopnea is "inability to breathe except in an upright position (as in congestive heart failure).” Webster’s Third New International Dictionary 1594-95.
. Dyspnea is “difficult or labored respiration.” Webster's Third New International Dictionary III.