Brief Answer:
Thyroid

Detailed Answer:
Evaluation of a low TSH requires systematic investigation.

Subclinical hyperthyroidism is certainly one of the causes for this but other possibilities too need to be considered.

For example, prednisone can lower the TSH as well. So if your TSH was checked close to the time you were on prednisone, it can be due to that also. Moreover, once the TSH has been suppressed it can take some time to rise back into the normal range.
So even if you had a TSH check few days after stopping prednisone, it would likely still be low for several weeks.

I would like to believe you must have had your thyroid hormone levels checked. I am referring to Total T3 and Free T4. They were probably normal in your case and that is how a diagnosis of subclinical hyperthyroidism must have been made.
Because otherwise other diagnoses are possible.
For instance, if thyroid hormone levels were also low or low-normal, then a condition called Secondary Hypothyroidism needs to be ruled out.
In such a situation, when I see someone like you in my practice, I order the following tests in addition to a thorough physical examination:
CBC
CMP
Prolactin
FSH
HbA1c


After a compressive assessment, when the diagnosis of subclinical (and primary) hyperthyroidism is confirmed, then I typically order XXXXXXX 123 uptake and scan. It is a test that helps to determine the cause of hyperthyroidism i.e. 'diffuse toxic goitre' (whole gland overactive), 'toxic multi nodular goiter' (multiple nodules are overactive , 'AFTN = Autonomously Functioning Thyroid nodule '(one solitary nodule is overactive) or 'transient thyroiditis' ( i.e. temporary bruising of the thyroid that typically recovers on its own within couple months and requires no treatment with methimazole.)

The scan that you have undergone is likely a Thyroid Ultrasound. It is not the same as the XXXXXXX 123 uptake and scan that I have mentioned above.
You will be best served by seeing an endocrinologist in person

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Brief Answer:
Follow up

Detailed Answer:
May I re-emphasise that methimazole is used only in specific types of thyroid conditions. Some, such as thyroiditis, do not require methimazole in the first place.

Now once a clear diagnosis has been established as to the cause of your low TSH, and it is one of those that responds to methimazole then the answer to your question is that
'whether or not methimazole can be withdrawn slowly and stopped depends on the cause of the hyperthyroidism'

So if the cause is

1 Graves' disease: typically methimazole is continued for 12 to 18 months before attempts to slowly stop are made

2 One or more overactive nodules found on the XXXXXXX 123 scan: these typically do not improve over time and methimazole is often continued long term. That is why other treatment options such as radio-ablation and surgery are considered for these two types of thyroid conditions.

As you can see, it is not as simple as your TSH rising back up into the range of normal that indicates the possibility of stopping medication. If you have one of the thyroid conditions described in 1) and 2) above , then your TSH has normalised because you have taken methimazole. And if you stop it, then it is likely to go back to what it was to begin with.

However, thyroid matters can be complex. A correct diagnosis is critical before you can discuss appropriate treatment options.

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