Nouns ending in a consonant followed by -y change the -y to -ies in the plural form.
Thoroughly is regular. It follows the -ies Conversion rule.
Use plural verbs: "The thoroughlies are ready" (not "is ready").
Yes. The plural thoroughlies is standard across all English variants.
The word thoroughly follows the standard English pluralization rule. When forming the plural, thoroughly becomes thoroughlies. This follows the -ies Conversion pattern, which is one of the most common plural rules in English.
Understanding why thoroughly becomes thoroughlies helps with spelling and pronunciation. Nouns ending in a consonant followed by -y change the -y to -ies in the plural form.
When using thoroughlies in writing, remember that it functions as a plural noun in sentences. It pairs with plural verbs and pronouns. For example: "The thoroughlies are ready" (not "is ready"). This subject-verb agreement is essential for grammatical correctness.