An estimated 58.3 million Americans receive a Social Security check each month, according to the Social Security Administration (SSA), and for some, it represents their primary source of retirement income. If you're banking on Social Security to supplement what you've saved in a 401(k), IRA or another qualified retirement plan, you may be in for a shock once your first payment arrives. You thought you were getting one amount, but the amount in your bank account is lower.

If you recently started receiving Social Security, here are three reasons why you may be getting less than you expected. (See our Introduction to Social Security.) 

An Offset Shrank Your Social Security Check

One potential scenario that could result in lower Social Security benefits is an offset. That's when someone you owe money to makes a claim against your benefits. If it's determined that the debt belongs to you, the Social Security Administration will reduce your benefits each month by a certain amount until what you owe is repaid. SSA regulations protect the first $750 in benefits you receive. Examples of debts that could result in an offset include:

You could also be subject to an offset if you're receiving Social Security benefits before you reach full retirement age and you continue to work. For 2017, beneficiaries who are working but haven't hit full retirement age will see their benefits decrease by one dollar for every two dollars they earn over $16,920. Once an offset for a debt is satisfied, or you reach your normal retirement age, you'll receive your full benefit amount. Meantime, you have to deal with the temporary shortfall. 

You Took Benefits Early

For most people, full retirement age is either 66 or 67 but it's possible to begin taking Social Security as early as 62. While that can give you some financial relief if you're strapped for cash, there's a trade-off because your benefits automatically go down. A study from the Nationwide Retirement Institute found that 29% of future retirees say they plan to apply for benefits early. In that same study, 24% of recent retirees said their Social Security check was smaller than they anticipated. (For more, read Tips on When to Claim Social Security.) 

How much can taking benefits early really cost you? Let's say your normal retirement age is 67, but you decide to apply for Social Security when you turn 62. Because you're taking benefits for an extra 60 months, your Social Security check would be reduced by 30%. If you're entitled to $1,000 a month, you'd only get $700. That's a pretty significant chunk of money to give up – and that check will be lower for life. If you're thinking of getting benefits early, it pays to crunch the numbers to see how much you stand to lose by doing so. 

Your Medicare Premiums Are Higher than You Expected

Seniors are eligible to enroll in Medicare in the year they turn 65. If you sign up for Medicare Part B, your premiums are deducted from your Social Security benefits. For 2018, the standard monthly premium is set at $134. However, it's entirely possible that you could end up paying more if you fall into a higher tax bracket. For certain high-income earners, premiums are equivalent to 30, 50, 65 or 80% of the total cost of coverage. If you file an individual return, for example, and your income is higher than $85,000 and up to $107,000, you will pay $187.50; if it's above $160,000 up to $214,000 it's $348.30. Click here for details.

"Most retirees have Medicare Part B premiums of $134 per month deducted from their Social Security check. However, some high-income retirees are shocked to find that their premiums can be as high as $428.60 per month," says James B. Twining, CFP, CEO of Financial Plan, Inc., Bellingham, Wash. "If your income has recently dropped, you may appeal to the SSA for a lower premium. The IRS may be providing the SSA with older data that needs to be updated."

If you expect your income to go up instead of down in retirement because you sell off a high-value asset or you decide to start a business, that could substantially impact what you get from Social Security. Your benefits could dwindle even further if you have Medicare Parts A and B and you're also paying a separate premium for a supplemental policy, also called a Medigap plan.

The Bottom Line

Relying on Social Security to see you through retirement can put you on thin ice financially. It becomes even trickier when you're getting less money than you'd budgeted to receive. Taking the time to clear up any outstanding debts, weighing the cost of taking benefits early and looking at how your income stands to affect your benefits can help you avoid any surprises once your Social Security checks start rolling in.